diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0a762dd..53cd7b5 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -113,6 +113,7 @@ add_executable(secsgem_tests
   tests/test_carriers.cpp
   tests/test_carrier_persistence.cpp
   tests/test_substrates.cpp
+  tests/test_substrate_persistence.cpp
   tests/test_ept.cpp
   tests/test_cem_objects.cpp
   tests/test_modules.cpp
diff --git a/include/secsgem/gem/store/substrates.hpp b/include/secsgem/gem/store/substrates.hpp
index 8dbb074..16bad54 100644
--- a/include/secsgem/gem/store/substrates.hpp
+++ b/include/secsgem/gem/store/substrates.hpp
@@ -2,10 +2,15 @@
 
 #include <chrono>
 #include <cstdint>
+#include <cstdio>
+#include <filesystem>
+#include <fstream>
 #include <functional>
 #include <map>
 #include <memory>
+#include <optional>
 #include <string>
+#include <system_error>
 #include <utility>
 #include <variant>
 #include <vector>
@@ -34,6 +39,7 @@ struct Substrate {
   std::string location;       // free-form: equipment-defined module name
   std::vector<SubstrateHistoryEntry> history;
   std::unique_ptr<SubstrateStateMachine> fsm;
+  std::filesystem::path journal_path;  // empty when persistence disabled
 };
 
 class SubstrateStore {
@@ -67,22 +73,9 @@ class SubstrateStore {
   CreateResult create(std::string substid, std::string carrierid = "",
                       uint8_t slot = 0) {
     if (subs_.count(substid)) return CreateResult::Denied_AlreadyExists;
-    auto fsm = std::make_unique<SubstrateStateMachine>();
-    const std::string id = substid;
-    fsm->set_location_handler(
-        [this, id](SubstrateState f, SubstrateState t, SubstrateEvent e) {
-          record_history(id, t, SubstrateProcessingState::NoState, e);
-          if (on_loc_) on_loc_(id, f, t, e);
-        });
-    fsm->set_processing_handler(
-        [this, id](SubstrateProcessingState f, SubstrateProcessingState t,
-                   SubstrateProcessingEvent e) {
-          record_history(id, SubstrateState::NoState, t, e);
-          if (on_proc_) on_proc_(id, f, t, e);
-        });
-    subs_.emplace(substid,
-                  Substrate{substid, std::move(carrierid), slot, "",
-                            {}, std::move(fsm)});
+    install_(substid, carrierid, slot, "", std::nullopt, std::nullopt,
+             std::nullopt);
+    if (persistent_) write_record_(substid);
     return CreateResult::Created;
   }
 
@@ -101,14 +94,27 @@ class SubstrateStore {
     auto* s = get(id);
     if (!s) return false;
     if (!new_location.empty()) s->location = std::move(new_location);
-    return s->fsm->on_location_event(e);
+    bool ok = s->fsm->on_location_event(e);
+    if (persistent_) write_record_(id);
+    return ok;
   }
   bool fire_processing_event(const std::string& id, SubstrateProcessingEvent e) {
     auto* s = get(id);
-    return s && s->fsm->on_processing_event(e);
+    if (!(s && s->fsm->on_processing_event(e))) return false;
+    if (persistent_) write_record_(id);
+    return true;
   }
 
-  bool remove(const std::string& id) { return subs_.erase(id) > 0; }
+  bool remove(const std::string& id) {
+    auto it = subs_.find(id);
+    if (it == subs_.end()) return false;
+    if (persistent_ && !it->second.journal_path.empty()) {
+      std::error_code ec;
+      std::filesystem::remove(it->second.journal_path, ec);
+    }
+    subs_.erase(it);
+    return true;
+  }
   std::size_t size() const { return subs_.size(); }
   std::vector<std::string> ids() const {
     std::vector<std::string> out;
@@ -122,7 +128,155 @@ class SubstrateStore {
     return s ? &s->history : nullptr;
   }
 
+  // ---- Persistence ----------------------------------------------------
+  // Records *current state* per substrate.  History is not journaled —
+  // it's a transient in-memory ring buffer and rebuilding it from
+  // replayed state would be misleading.
+  void enable_persistence(std::filesystem::path dir) {
+    namespace fs = std::filesystem;
+    journal_dir_ = std::move(dir);
+    persistent_ = true;
+    std::error_code ec;
+    fs::create_directories(journal_dir_, ec);
+
+    std::vector<std::pair<uint64_t, fs::path>> existing;
+    for (auto& entry : fs::directory_iterator(journal_dir_, ec)) {
+      if (!entry.is_regular_file()) continue;
+      const auto& p = entry.path();
+      if (p.extension() != ".sub") continue;
+      uint64_t seq = 0;
+      try { seq = std::stoull(p.stem().string()); }
+      catch (...) { continue; }
+      existing.emplace_back(seq, p);
+    }
+    std::sort(existing.begin(), existing.end(),
+              [](const auto& a, const auto& b) { return a.first < b.first; });
+
+    for (auto& [seq, path] : existing) {
+      auto rec = load_record_(path);
+      if (!rec) { fs::remove(path, ec); continue; }
+      install_(rec->substid, rec->carrierid, rec->slot, rec->location,
+               rec->loc_state, rec->proc_state, rec->id_state);
+      auto* s = get(rec->substid);
+      if (s) s->journal_path = path;
+      if (seq >= next_seq_) next_seq_ = seq + 1;
+    }
+  }
+
+  bool persistence_enabled() const { return persistent_; }
+  std::filesystem::path journal_dir() const { return journal_dir_; }
+
  private:
+  struct Record {
+    std::string substid;
+    std::string carrierid;
+    uint8_t slot;
+    std::string location;
+    SubstrateState           loc_state;
+    SubstrateProcessingState proc_state;
+    SubstrateIDStatus        id_state;
+  };
+
+  // Substrate record:
+  //   [u8 magic = 0xC6][u8 version = 1]
+  //   [u8 loc_state][u8 proc_state][u8 id_state][u8 slot]
+  //   [u16 substid_len][substid_len × u8]
+  //   [u16 carrierid_len][carrierid_len × u8]
+  //   [u16 location_len][location_len × u8]
+  static constexpr uint8_t kMagic   = 0xC6;
+  static constexpr uint8_t kVersion = 0x01;
+
+  void install_(const std::string& substid, const std::string& carrierid,
+                uint8_t slot, const std::string& location,
+                std::optional<SubstrateState> loc,
+                std::optional<SubstrateProcessingState> proc,
+                std::optional<SubstrateIDStatus> id) {
+    auto fsm = std::make_unique<SubstrateStateMachine>();
+    if (loc || proc || id) {
+      fsm->restore_state(loc.value_or(SubstrateState::AtSource),
+                         proc.value_or(SubstrateProcessingState::NeedsProcessing),
+                         id.value_or(SubstrateIDStatus::NotConfirmed));
+    }
+    const std::string keyed = substid;
+    fsm->set_location_handler(
+        [this, keyed](SubstrateState f, SubstrateState t, SubstrateEvent e) {
+          record_history(keyed, t, SubstrateProcessingState::NoState, e);
+          if (on_loc_) on_loc_(keyed, f, t, e);
+        });
+    fsm->set_processing_handler(
+        [this, keyed](SubstrateProcessingState f, SubstrateProcessingState t,
+                      SubstrateProcessingEvent e) {
+          record_history(keyed, SubstrateState::NoState, t, e);
+          if (on_proc_) on_proc_(keyed, f, t, e);
+        });
+    subs_.emplace(substid,
+                  Substrate{substid, carrierid, slot, location, {},
+                            std::move(fsm), {}});
+  }
+
+  void write_record_(const std::string& substid) {
+    namespace fs = std::filesystem;
+    auto* s = get(substid);
+    if (!s) return;
+    if (s->journal_path.empty()) {
+      char namebuf[32];
+      std::snprintf(namebuf, sizeof(namebuf), "%010llu.sub",
+                    static_cast<unsigned long long>(next_seq_++));
+      s->journal_path = journal_dir_ / namebuf;
+    }
+    fs::path tmp = s->journal_path;
+    tmp += ".tmp";
+    std::ofstream out(tmp, std::ios::binary | std::ios::trunc);
+    if (!out) return;
+    uint8_t header[6] = {
+        kMagic, kVersion,
+        static_cast<uint8_t>(s->fsm->location_state()),
+        static_cast<uint8_t>(s->fsm->processing_state()),
+        static_cast<uint8_t>(s->fsm->id_state()),
+        s->slot};
+    out.write(reinterpret_cast<const char*>(header), sizeof(header));
+    auto write_str = [&](const std::string& str) {
+      uint16_t n = static_cast<uint16_t>(str.size());
+      uint8_t lb[2] = {static_cast<uint8_t>(n >> 8), static_cast<uint8_t>(n)};
+      out.write(reinterpret_cast<const char*>(lb), 2);
+      out.write(str.data(), static_cast<std::streamsize>(n));
+    };
+    write_str(s->substid);
+    write_str(s->carrierid);
+    write_str(s->location);
+    out.flush();
+    out.close();
+    std::error_code ec;
+    fs::rename(tmp, s->journal_path, ec);
+    if (ec) fs::remove(tmp, ec);
+  }
+
+  static std::optional<Record> load_record_(const std::filesystem::path& p) {
+    std::ifstream in(p, std::ios::binary);
+    if (!in) return std::nullopt;
+    uint8_t header[6];
+    in.read(reinterpret_cast<char*>(header), sizeof(header));
+    if (!in || header[0] != kMagic || header[1] != kVersion) return std::nullopt;
+    Record r;
+    r.loc_state  = static_cast<SubstrateState>(header[2]);
+    r.proc_state = static_cast<SubstrateProcessingState>(header[3]);
+    r.id_state   = static_cast<SubstrateIDStatus>(header[4]);
+    r.slot       = header[5];
+    auto read_str = [&](std::string& out) {
+      uint8_t lb[2];
+      in.read(reinterpret_cast<char*>(lb), 2);
+      if (!in) return false;
+      uint16_t n = static_cast<uint16_t>((uint16_t(lb[0]) << 8) | lb[1]);
+      out.resize(n);
+      in.read(out.data(), n);
+      return static_cast<bool>(in) || n == 0;
+    };
+    if (!read_str(r.substid))   return std::nullopt;
+    if (!read_str(r.carrierid)) return std::nullopt;
+    if (!read_str(r.location))  return std::nullopt;
+    return r;
+  }
+
   template <typename Event>
   void record_history(const std::string& id, SubstrateState loc_to,
                       SubstrateProcessingState proc_to, Event ev) {
@@ -146,6 +300,9 @@ class SubstrateStore {
   LocationChangeHandler   on_loc_;
   ProcessingChangeHandler on_proc_;
   std::size_t history_limit_ = 256;
+  bool persistent_ = false;
+  std::filesystem::path journal_dir_;
+  uint64_t next_seq_ = 0;
 };
 
 }  // namespace secsgem::gem
diff --git a/include/secsgem/gem/substrate_state.hpp b/include/secsgem/gem/substrate_state.hpp
index 1f0fa02..ef8bd4c 100644
--- a/include/secsgem/gem/substrate_state.hpp
+++ b/include/secsgem/gem/substrate_state.hpp
@@ -127,6 +127,15 @@ class SubstrateStateMachine {
   bool on_processing_event(SubstrateProcessingEvent e);
   bool on_id_event(SubstrateIDEvent e);
 
+  // Direct state-restore (persistence replay only).  Bypasses the
+  // transition tables and does NOT fire change handlers.
+  void restore_state(SubstrateState loc, SubstrateProcessingState proc,
+                     SubstrateIDStatus id) {
+    loc_  = loc;
+    proc_ = proc;
+    id_   = id;
+  }
+
  private:
   SubstrateTable           loc_table_;
   SubstrateProcessingTable proc_table_;
diff --git a/tests/test_substrate_persistence.cpp b/tests/test_substrate_persistence.cpp
new file mode 100644
index 0000000..8ff25d6
--- /dev/null
+++ b/tests/test_substrate_persistence.cpp
@@ -0,0 +1,186 @@
+// Persistence tests for SubstrateStore.  Mirrors test_carrier_persistence:
+// enable a journal directory, drive every FSM axis, "restart" by
+// constructing a fresh store pointed at the same directory, assert
+// replay reproduces in-memory state.
+
+#include <doctest/doctest.h>
+
+#include <filesystem>
+#include <fstream>
+#include <random>
+#include <string>
+
+#include "secsgem/gem/store/substrates.hpp"
+
+using namespace secsgem::gem;
+namespace fs = std::filesystem;
+
+namespace {
+fs::path scratch_dir(const char* tag) {
+  std::random_device rd;
+  auto p = fs::temp_directory_path() /
+           (std::string("secsgem-substrate-") + tag + "-" +
+            std::to_string(rd()));
+  fs::remove_all(p);
+  fs::create_directories(p);
+  return p;
+}
+std::size_t count_with_ext(const fs::path& dir, const std::string& ext) {
+  std::size_t n = 0;
+  std::error_code ec;
+  for (auto& e : fs::directory_iterator(dir, ec)) {
+    if (e.is_regular_file() && e.path().extension() == ext) ++n;
+  }
+  return n;
+}
+}  // namespace
+
+TEST_CASE("SubstrateStore persistence: replay reproduces all three axes") {
+  auto dir = scratch_dir("replay");
+  {
+    SubstrateStore s;
+    s.enable_persistence(dir);
+    REQUIRE(s.create("W-1", "CAR-A", 1) ==
+            SubstrateStore::CreateResult::Created);
+    REQUIRE(s.create("W-2", "CAR-A", 2) ==
+            SubstrateStore::CreateResult::Created);
+
+    // Drive each axis for W-1: location, processing, id.
+    REQUIRE(s.fire_location_event("W-1", SubstrateEvent::Acquire, "MOD-A"));
+    REQUIRE(s.fire_processing_event("W-1",
+                                     SubstrateProcessingEvent::StartProcessing));
+    // The store has no fire_id_event helper, but mutating the FSM does
+    // trigger handlers; persistence is only refreshed via the store
+    // entrypoints — for this test we use the FSM directly and then call
+    // a location event to force a journal rewrite.
+    REQUIRE(s.get("W-1")->fsm->on_id_event(SubstrateIDEvent::Read));
+    REQUIRE(s.get("W-1")->fsm->on_id_event(SubstrateIDEvent::Confirm));
+    REQUIRE(s.fire_location_event("W-1", SubstrateEvent::Release, "DEST"));
+    REQUIRE(s.fire_processing_event("W-1",
+                                     SubstrateProcessingEvent::EndProcessing));
+
+    CHECK(count_with_ext(dir, ".sub") == 2);
+  }
+  {
+    SubstrateStore s;
+    s.enable_persistence(dir);
+    REQUIRE(s.has("W-1"));
+    REQUIRE(s.has("W-2"));
+    const auto* w1 = s.get("W-1");
+    CHECK(w1->carrierid == "CAR-A");
+    CHECK(w1->slot == 1);
+    CHECK(w1->location == "DEST");
+    CHECK(w1->fsm->location_state() == SubstrateState::AtDestination);
+    CHECK(w1->fsm->processing_state() == SubstrateProcessingState::Processed);
+    CHECK(w1->fsm->id_state() == SubstrateIDStatus::Confirmed);
+    const auto* w2 = s.get("W-2");
+    CHECK(w2->fsm->location_state() == SubstrateState::AtSource);
+    CHECK(w2->fsm->processing_state() == SubstrateProcessingState::NeedsProcessing);
+  }
+  fs::remove_all(dir);
+}
+
+TEST_CASE("SubstrateStore persistence: each terminal processing state round-trips") {
+  auto dir = scratch_dir("terminal");
+  {
+    SubstrateStore s;
+    s.enable_persistence(dir);
+    // Abort/Stop/Reject are only legal from InProcess per E90 §6.4.4,
+    // so drive Start first for those three.
+    auto drive_into = [&](const std::string& id, uint8_t slot,
+                          SubstrateProcessingEvent terminal) {
+      REQUIRE(s.create(id, "CAR", slot) ==
+              SubstrateStore::CreateResult::Created);
+      REQUIRE(s.fire_processing_event(id,
+                                       SubstrateProcessingEvent::StartProcessing));
+      REQUIRE(s.fire_processing_event(id, terminal));
+    };
+    drive_into("W-AB", 1, SubstrateProcessingEvent::Abort);
+    drive_into("W-ST", 2, SubstrateProcessingEvent::Stop);
+    drive_into("W-RJ", 3, SubstrateProcessingEvent::Reject);
+
+    // Lost and Skip are reachable from NeedsProcessing.
+    REQUIRE(s.create("W-LO", "CAR", 4) == SubstrateStore::CreateResult::Created);
+    REQUIRE(s.fire_processing_event("W-LO", SubstrateProcessingEvent::ReportLost));
+
+    REQUIRE(s.create("W-SK", "CAR", 5) == SubstrateStore::CreateResult::Created);
+    REQUIRE(s.fire_processing_event("W-SK", SubstrateProcessingEvent::Skip));
+  }
+  {
+    SubstrateStore s;
+    s.enable_persistence(dir);
+    CHECK(s.get("W-AB")->fsm->processing_state() ==
+          SubstrateProcessingState::Aborted);
+    CHECK(s.get("W-ST")->fsm->processing_state() ==
+          SubstrateProcessingState::Stopped);
+    CHECK(s.get("W-RJ")->fsm->processing_state() ==
+          SubstrateProcessingState::Rejected);
+    CHECK(s.get("W-LO")->fsm->processing_state() ==
+          SubstrateProcessingState::Lost);
+    CHECK(s.get("W-SK")->fsm->processing_state() ==
+          SubstrateProcessingState::Skipped);
+  }
+  fs::remove_all(dir);
+}
+
+TEST_CASE("SubstrateStore persistence: remove deletes journal file") {
+  auto dir = scratch_dir("remove");
+  SubstrateStore s;
+  s.enable_persistence(dir);
+  REQUIRE(s.create("W-A") == SubstrateStore::CreateResult::Created);
+  REQUIRE(s.create("W-B") == SubstrateStore::CreateResult::Created);
+  CHECK(count_with_ext(dir, ".sub") == 2);
+  REQUIRE(s.remove("W-A"));
+  CHECK(count_with_ext(dir, ".sub") == 1);
+
+  SubstrateStore s2;
+  s2.enable_persistence(dir);
+  CHECK_FALSE(s2.has("W-A"));
+  CHECK(s2.has("W-B"));
+  fs::remove_all(dir);
+}
+
+TEST_CASE("SubstrateStore persistence: corrupt record dropped, others survive") {
+  auto dir = scratch_dir("corrupt");
+  {
+    SubstrateStore s;
+    s.enable_persistence(dir);
+    REQUIRE(s.create("W-OK") == SubstrateStore::CreateResult::Created);
+  }
+  // Wrong magic.
+  {
+    std::ofstream f(dir / "9999999999.sub", std::ios::binary);
+    const char garbage[] = {0x00, 0x00, 0x00};
+    f.write(garbage, sizeof(garbage));
+  }
+  SubstrateStore s;
+  s.enable_persistence(dir);
+  CHECK(s.has("W-OK"));
+  CHECK(s.size() == 1);
+  CHECK(count_with_ext(dir, ".sub") == 1);
+  fs::remove_all(dir);
+}
+
+TEST_CASE("SubstrateStore: history is NOT journaled (transient by design)") {
+  auto dir = scratch_dir("history");
+  {
+    SubstrateStore s;
+    s.enable_persistence(dir);
+    REQUIRE(s.create("W-H") == SubstrateStore::CreateResult::Created);
+    REQUIRE(s.fire_location_event("W-H", SubstrateEvent::Acquire, "MOD"));
+    REQUIRE(s.fire_processing_event("W-H",
+                                     SubstrateProcessingEvent::StartProcessing));
+    CHECK(s.history("W-H")->size() == 2);
+  }
+  {
+    SubstrateStore s;
+    s.enable_persistence(dir);
+    REQUIRE(s.has("W-H"));
+    // State survived but history is empty post-replay — current state
+    // is what's durable, not the journey to it.
+    CHECK(s.history("W-H")->size() == 0);
+    CHECK(s.get("W-H")->fsm->processing_state() ==
+          SubstrateProcessingState::InProcess);
+  }
+  fs::remove_all(dir);
+}