diff --git a/src/core/adapter.ts b/src/core/adapter.ts
new file mode 100644
index 0000000..0b199a3
--- /dev/null
+++ b/src/core/adapter.ts
@@ -0,0 +1,118 @@
+/**
+ * Ledger Adapter Interface
+ *
+ * Defines the contract that any ORM adapter must satisfy.
+ * TypeScript structural typing handles conformance -- no runtime enforcement needed.
+ *
+ * Generic parameters:
+ * - TDb: The database instance type (e.g., DrizzleSQLiteDatabase)
+ * - TAuditTable: The audit log table reference type (e.g., typeof auditLog)
+ * - TQueryFilter: The query filter/condition type returned by filter functions (e.g., SQL)
+ */
+
+import type { AuditLogEntry } from "./types.js";
+import type { PurgeConfig, PurgeResult } from "./gdpr.js";
+
+/**
+ * Configuration for audited database wrapping.
+ * Adapter implementations may extend this with ORM-specific options.
+ */
+export interface AuditedDbConfig {
+  /** Tables to exclude from soft-delete (will hard delete) */
+  hardDeleteTables?: string[];
+}
+
+/**
+ * The contract every ORM adapter must implement.
+ *
+ * Each method corresponds to a capability the adapter provides.
+ * Adapters are free to expose additional ORM-specific helpers
+ * (e.g., logInsert, logUpdate, AuditLogger) beyond this contract.
+ */
+export interface LedgerAdapter<TDb, TAuditTable, TQueryFilter> {
+  // -- Query filters --
+
+  /**
+   * Return a filter condition that excludes soft-deleted records.
+   *
+   * @param table - A table with a deletedAt column
+   * @returns A query filter for non-deleted records
+   */
+  notDeleted(table: { deletedAt: unknown }): TQueryFilter;
+
+  /**
+   * Return a filter condition that includes only soft-deleted records.
+   *
+   * @param table - A table with a deletedAt column
+   * @returns A query filter for deleted records only
+   */
+  onlyDeleted(table: { deletedAt: unknown }): TQueryFilter;
+
+  // -- Audit operations --
+
+  /**
+   * Persist an audit log entry to the database.
+   *
+   * @param db - The database instance
+   * @param auditTable - The audit log table reference
+   * @param entry - The audit entry to persist
+   */
+  insertAuditEntry(db: TDb, auditTable: TAuditTable, entry: AuditLogEntry): Promise<void>;
+
+  /**
+   * Retrieve the full audit history for a specific record, newest first.
+   *
+   * @param db - The database instance
+   * @param auditTable - The audit log table reference
+   * @param tableName - Name of the table the record belongs to
+   * @param recordId - Primary key of the record
+   * @returns Audit entries ordered by creation time descending
+   */
+  getRecordHistory(
+    db: TDb,
+    auditTable: TAuditTable,
+    tableName: string,
+    recordId: string,
+  ): Promise<AuditLogEntry[]>;
+
+  // -- GDPR --
+
+  /**
+   * Anonymize all audit data for a given user.
+   * Preserves audit trail structure but removes PII.
+   *
+   * @param db - The database instance
+   * @param auditTable - The audit log table reference
+   * @param userId - The user whose data should be purged
+   * @param config - Optional purge configuration
+   * @returns Result with count of affected records
+   */
+  purgeUserData(
+    db: TDb,
+    auditTable: TAuditTable,
+    userId: string,
+    config?: PurgeConfig,
+  ): Promise<PurgeResult>;
+
+  /**
+   * Check whether a user's data has already been purged.
+   *
+   * @param db - The database instance
+   * @param auditTable - The audit log table reference
+   * @param userId - The user to check
+   * @returns true if no audit entries remain with this userId
+   */
+  isUserDataPurged(db: TDb, auditTable: TAuditTable, userId: string): Promise<boolean>;
+
+  // -- Soft-delete automation --
+
+  /**
+   * Wrap a database instance so that delete() calls are automatically
+   * converted to soft-delete for tables with a deletedAt column.
+   *
+   * @param db - The database instance to wrap
+   * @param config - Optional configuration
+   * @returns The wrapped database instance (same type as input)
+   */
+  createAuditedDb(db: TDb, config?: AuditedDbConfig): TDb;
+}
diff --git a/src/core/index.ts b/src/core/index.ts
index 15a1c3a..c108bae 100644
--- a/src/core/index.ts
+++ b/src/core/index.ts
@@ -16,6 +16,9 @@ export type {
   SoftDeleteResult,
 } from "./types.js";
 
+// Adapter interface
+export type { LedgerAdapter } from "./adapter.js";
+
 // Context
 export {
   createLedgerContext,
diff --git a/test/drizzle/adapter-compat.test.ts b/test/drizzle/adapter-compat.test.ts
new file mode 100644
index 0000000..08f828f
--- /dev/null
+++ b/test/drizzle/adapter-compat.test.ts
@@ -0,0 +1,92 @@
+/**
+ * Type-level test: Drizzle adapter satisfies LedgerAdapter interface.
+ *
+ * This file verifies structural compatibility at compile time.
+ * If the Drizzle adapter drifts from the interface, this file
+ * will produce a type error -- no runtime assertions needed.
+ */
+
+import type { SQL } from "drizzle-orm";
+import { describe, expect, test } from "vitest";
+import type { LedgerAdapter } from "../../src/core/adapter.js";
+import { insertAuditEntry, getRecordHistory } from "../../src/drizzle/audit.js";
+import { createAuditedDb } from "../../src/drizzle/db.js";
+import { purgeUserData, isUserDataPurged } from "../../src/drizzle/gdpr.js";
+import { notDeleted, onlyDeleted } from "../../src/drizzle/soft-delete.js";
+import type { AuditLog } from "../../src/drizzle/schema/sqlite.js";
+
+// -- Structural db type covering all Drizzle adapter function requirements --
+// Each function uses a slightly different duck-typed db shape. This union
+// represents the minimal surface a Drizzle db instance exposes.
+type DrizzleDb = {
+  insert: (table: AuditLog) => {
+    values: (entry: unknown) => { execute: () => Promise<unknown> };
+  };
+  select: () => {
+    from: (table: AuditLog) => {
+      where: (condition: unknown) => {
+        orderBy: (order: unknown) => Promise<Array<AuditLog["$inferSelect"]>>;
+      };
+    };
+  };
+  update: (table: unknown) => {
+    set: (values: Record<string, unknown>) => {
+      where: (condition: unknown) => {
+        returning: () => {
+          execute: () => Promise<unknown>;
+        };
+        execute: () => Promise<unknown>;
+      };
+    };
+  };
+  delete: (table: unknown) => {
+    where: (condition: unknown) => {
+      returning: () => {
+        execute: () => Promise<unknown>;
+      };
+      execute: () => Promise<unknown>;
+    };
+  };
+};
+
+// -- Build the adapter object from standalone Drizzle functions --
+const drizzleAdapter = {
+  notDeleted,
+  onlyDeleted,
+  insertAuditEntry,
+  getRecordHistory,
+  purgeUserData,
+  isUserDataPurged,
+  createAuditedDb,
+} satisfies LedgerAdapter<DrizzleDb, AuditLog, SQL>;
+
+// Alternate verification: assignment check
+const _check: LedgerAdapter<DrizzleDb, AuditLog, SQL> = drizzleAdapter;
+
+describe("LedgerAdapter interface", () => {
+  test("Drizzle adapter structurally satisfies the interface", () => {
+    // The real verification is the type-level satisfies/assignment above.
+    // If this file compiles, the adapter conforms.
+    expect(drizzleAdapter).toBeDefined();
+    expect(drizzleAdapter.notDeleted).toBeTypeOf("function");
+    expect(drizzleAdapter.onlyDeleted).toBeTypeOf("function");
+    expect(drizzleAdapter.insertAuditEntry).toBeTypeOf("function");
+    expect(drizzleAdapter.getRecordHistory).toBeTypeOf("function");
+    expect(drizzleAdapter.purgeUserData).toBeTypeOf("function");
+    expect(drizzleAdapter.isUserDataPurged).toBeTypeOf("function");
+    expect(drizzleAdapter.createAuditedDb).toBeTypeOf("function");
+  });
+
+  test("adapter object has exactly the interface methods", () => {
+    const keys = Object.keys(drizzleAdapter).sort();
+    expect(keys).toEqual([
+      "createAuditedDb",
+      "getRecordHistory",
+      "insertAuditEntry",
+      "isUserDataPurged",
+      "notDeleted",
+      "onlyDeleted",
+      "purgeUserData",
+    ]);
+  });
+});