diff --git a/src/event_log.rs b/src/event_log.rs
index 693a1ef..63f82af 100644
--- a/src/event_log.rs
+++ b/src/event_log.rs
@@ -1 +1,288 @@
-// Placeholder for upcoming event_log module.
+use crate::domain::{unix_secs_now, RuntimeState, POLICY_SCHEMA_VERSION};
+use anyhow::{anyhow, Context, Result};
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+use sha2::{Digest, Sha256};
+use std::fs::{self, File, OpenOptions};
+use std::io::{BufRead, BufReader, Write};
+use std::path::{Path, PathBuf};
+
+#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "snake_case")]
+pub enum EventType {
+    CommitmentCreated,
+    RuntimeTransition,
+    CommitmentTransition,
+    PolicyApplied,
+    EvidenceObserved,
+    ViolationObserved,
+    TransitionStarted,
+    ReviewCompleted,
+}
+
+#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
+pub struct EventRecord {
+    pub schema_version: u16,
+    pub sequence: u64,
+    pub timestamp_unix_secs: u64,
+    pub event_type: EventType,
+    pub commitment_id: Option<String>,
+    pub runtime_state: RuntimeState,
+    pub payload_json: Value,
+    pub previous_hash: Option<String>,
+    pub hash: String,
+}
+
+#[derive(Debug)]
+pub struct EventLog {
+    path: PathBuf,
+    next_sequence: u64,
+    previous_hash: Option<String>,
+}
+
+impl EventLog {
+    pub fn open(path: impl AsRef<Path>) -> Result<Self> {
+        let path = path.as_ref().to_path_buf();
+        if let Some(parent) = path.parent() {
+            fs::create_dir_all(parent).with_context(|| {
+                format!("create parent directories for event log {}", path.display())
+            })?;
+        }
+
+        let mut next_sequence = 1;
+        let mut previous_hash = None;
+
+        if path.exists() {
+            let file =
+                File::open(&path).with_context(|| format!("open event log {}", path.display()))?;
+            for (line_index, line) in BufReader::new(file).lines().enumerate() {
+                let line_number = line_index + 1;
+                let line = line.with_context(|| {
+                    format!("read event log line {line_number} from {}", path.display())
+                })?;
+                if line.trim().is_empty() {
+                    continue;
+                }
+
+                let record: EventRecord = serde_json::from_str(&line).with_context(|| {
+                    format!("parse event log line {line_number} from {}", path.display())
+                })?;
+                validate_record(&record, next_sequence, previous_hash.as_deref())
+                    .map_err(|err| anyhow!("validate event log line {line_number}: {err}"))?;
+
+                next_sequence += 1;
+                previous_hash = Some(record.hash);
+            }
+        }
+
+        Ok(Self {
+            path,
+            next_sequence,
+            previous_hash,
+        })
+    }
+
+    pub fn append(
+        &mut self,
+        event_type: EventType,
+        runtime_state: RuntimeState,
+        commitment_id: Option<String>,
+        payload_json: Value,
+    ) -> Result<EventRecord> {
+        if let Some(parent) = self.path.parent() {
+            fs::create_dir_all(parent).with_context(|| {
+                format!(
+                    "create parent directories for event log {}",
+                    self.path.display()
+                )
+            })?;
+        }
+
+        let mut record = EventRecord {
+            schema_version: POLICY_SCHEMA_VERSION,
+            sequence: self.next_sequence,
+            timestamp_unix_secs: unix_secs_now(),
+            event_type,
+            commitment_id,
+            runtime_state,
+            payload_json,
+            previous_hash: self.previous_hash.clone(),
+            hash: String::new(),
+        };
+        record.hash = record_hash(&record)?;
+
+        let mut file = OpenOptions::new()
+            .create(true)
+            .append(true)
+            .open(&self.path)
+            .with_context(|| format!("open event log for append {}", self.path.display()))?;
+        serde_json::to_writer(&mut file, &record)
+            .with_context(|| format!("serialize event log record to {}", self.path.display()))?;
+        file.write_all(b"\n")
+            .with_context(|| format!("write event log newline to {}", self.path.display()))?;
+        file.flush()
+            .with_context(|| format!("flush event log {}", self.path.display()))?;
+
+        self.next_sequence += 1;
+        self.previous_hash = Some(record.hash.clone());
+
+        Ok(record)
+    }
+}
+
+fn validate_record(
+    record: &EventRecord,
+    expected_sequence: u64,
+    expected_previous_hash: Option<&str>,
+) -> Result<()> {
+    if record.sequence != expected_sequence {
+        return Err(anyhow!(
+            "sequence mismatch: expected {}, found {}",
+            expected_sequence,
+            record.sequence
+        ));
+    }
+    if record.previous_hash.as_deref() != expected_previous_hash {
+        return Err(anyhow!(
+            "previous hash mismatch at sequence {}",
+            record.sequence
+        ));
+    }
+
+    let expected_hash = record_hash(record)?;
+    if record.hash != expected_hash {
+        return Err(anyhow!("hash mismatch at sequence {}", record.sequence));
+    }
+
+    Ok(())
+}
+
+fn record_hash(record: &EventRecord) -> Result<String> {
+    let mut hashable = record.clone();
+    hashable.hash.clear();
+    let bytes = serde_json::to_vec(&hashable).context("serialize event log record for hash")?;
+    Ok(hex::encode(Sha256::digest(bytes)))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::domain::RuntimeState;
+    use serde_json::json;
+    use std::fs;
+    use std::path::PathBuf;
+    use std::time::{SystemTime, UNIX_EPOCH};
+
+    fn temp_log_path(name: &str) -> PathBuf {
+        let nonce = SystemTime::now()
+            .duration_since(UNIX_EPOCH)
+            .unwrap()
+            .as_nanos();
+        std::env::temp_dir().join(format!(
+            "antidrift-event-log-{name}-{}-{nonce}.jsonl",
+            std::process::id()
+        ))
+    }
+
+    #[test]
+    fn appends_hash_chained_jsonl_records() {
+        let path = temp_log_path("append");
+        let mut log = EventLog::open(&path).unwrap();
+
+        let first = log
+            .append(
+                EventType::RuntimeTransition,
+                RuntimeState::Transition,
+                Some("commitment-1".to_string()),
+                json!({"from": "active", "to": "transition"}),
+            )
+            .unwrap();
+        let second = log
+            .append(
+                EventType::PolicyApplied,
+                RuntimeState::Active,
+                Some("commitment-1".to_string()),
+                json!({"policy_id": "policy-1"}),
+            )
+            .unwrap();
+
+        assert_eq!(first.sequence, 1);
+        assert_eq!(second.sequence, 2);
+        assert_eq!(second.previous_hash.as_deref(), Some(first.hash.as_str()));
+        assert_ne!(first.hash, second.hash);
+
+        let contents = fs::read_to_string(&path).unwrap();
+        assert_eq!(contents.lines().count(), 2);
+
+        fs::remove_file(path).unwrap();
+    }
+
+    #[test]
+    fn opening_existing_log_validates_chain_and_continues_sequence() {
+        let path = temp_log_path("reopen");
+        {
+            let mut log = EventLog::open(&path).unwrap();
+            log.append(
+                EventType::RuntimeTransition,
+                RuntimeState::Transition,
+                None,
+                json!({"from": "active"}),
+            )
+            .unwrap();
+        }
+
+        let mut reopened = EventLog::open(&path).unwrap();
+        let next = reopened
+            .append(
+                EventType::PolicyApplied,
+                RuntimeState::Active,
+                None,
+                json!({"policy_id": "policy-1"}),
+            )
+            .unwrap();
+
+        assert_eq!(next.sequence, 2);
+
+        fs::remove_file(path).unwrap();
+    }
+
+    #[test]
+    fn opening_malformed_json_log_fails() {
+        let path = temp_log_path("malformed");
+        fs::write(&path, "{not json}\n").unwrap();
+
+        let err = EventLog::open(&path).expect_err("malformed JSONL must fail");
+
+        assert!(err.to_string().contains("parse event log"));
+        fs::remove_file(path).unwrap();
+    }
+
+    #[test]
+    fn opening_tampered_log_chain_fails() {
+        let path = temp_log_path("tampered");
+        {
+            let mut log = EventLog::open(&path).unwrap();
+            log.append(
+                EventType::RuntimeTransition,
+                RuntimeState::Transition,
+                None,
+                json!({"from": "active"}),
+            )
+            .unwrap();
+            log.append(
+                EventType::PolicyApplied,
+                RuntimeState::Active,
+                None,
+                json!({"policy_id": "policy-1"}),
+            )
+            .unwrap();
+        }
+        let contents = fs::read_to_string(&path).unwrap();
+        fs::write(&path, contents.replace("\"sequence\":2", "\"sequence\":3")).unwrap();
+
+        let err = EventLog::open(&path).expect_err("tampered chain must fail");
+
+        assert!(err.to_string().contains("sequence mismatch"));
+        fs::remove_file(path).unwrap();
+    }
+}