diff --git a/lib/www/server/lib/qc/index.js b/lib/www/server/lib/qc/index.js
index 1657f43..7b444bf 100644
--- a/lib/www/server/lib/qc/index.js
+++ b/lib/www/server/lib/qc/index.js
@@ -5,7 +5,7 @@ const Cursor = require('pg-cursor');
 const { pool, setSurvey, transaction, fetchRow } = require('../db/connection')
 const { project, sequence, configuration, info } = require('../db')
 const flattenQCDefinitions = require('./flatten');
-const { projectLastModified, sequenceLastModified } = require('./last-modified');
+const { projectHash, sequenceHash } = require('./last-modified');
 
 const { runShotsQC, saveShotsQC } = require('./shots');
 const { runSequenceQCs, saveSequenceQCs } = require('./sequences');
@@ -35,15 +35,14 @@ async function main () {
 
 		if (!project.archived) {
 			const QCTstamp = new Date();
-			const projectTstamp = await projectLastModified(projectId);
-			const updatedOn = await info.get(projectId, "qc/updatedOn");
-			const lastQCTstamp = isNaN(new Date(updatedOn)) ? -Infinity : new Date(updatedOn);
-			console.log("QCTstamp", QCTstamp);
-			console.log("projectTstamp", projectTstamp);
-			console.log("lastQCTstamp", lastQCTstamp);
 
-			if (projectTstamp >= lastQCTstamp) {
-				console.log("projectTstamp >= lastQCTstamp", projectId, projectTstamp, lastQCTstamp, projectTstamp >= lastQCTstamp);
+			const currentQCHash = await projectHash(projectId);
+			const lastQCHash = await info.get(projectId, "qc/hash");
+			console.log("projectHash", projectHash);
+			console.log("lastQCHash", lastQCHash);
+
+			if (currentQCHash != lastQCHash) {
+				console.log("currentQCHash != lastQCHash", projectId, currentQCHash, lastQCHash);
 
 				// Fetch definitions and parameters
 				const { definitions, parameters } = await getProjectQCConfig(projectId);
@@ -60,14 +59,17 @@ async function main () {
 					// Run shot QCs
 					for (const seq of sequences) {
 						const sequenceNumber = seq.sequence;
-						const sequenceTstamp = await sequenceLastModified(projectId, sequenceNumber);
+						const sequenceCurrentHash = await sequenceHash(projectId, sequenceNumber);
+						const sequenceLastQCHash = seq.meta?.lastQCHash;
+						console.log("sequenceCurrentHash", sequenceCurrentHash);
+						console.log("sequenceLastQCHash", sequenceLastQCHash);
 
-						if (sequenceTstamp >= lastQCTstamp) {
+						if (sequenceCurrentHash != sequenceLastQCHash) {
 
 							const results = await runShotsQC(projectId, sequenceNumber, shotQCs, parameters);
 
 							await saveShotsQC(projectId, {[sequenceNumber]: results});
-// 							console.log("Saved", sequenceNumber);
+							await sequenceHash(projectId, sequenceNumber, sequenceCurrentHash);
 
 						} else {
 							console.log("NOT MODIFIED: SEQ", sequenceNumber);
@@ -80,7 +82,7 @@ async function main () {
 
 					// Run survey-wide QCs TODO maybe
 
-					await info.put(projectId, "qc", {updatedOn: QCTstamp}, {}, null);
+					await info.put(projectId, "qc", {updatedOn: QCTstamp, hash: currentQCHash}, {}, null);
 				}
 			}
 		}
diff --git a/lib/www/server/lib/qc/last-modified.js b/lib/www/server/lib/qc/last-modified.js
index 05c3de9..b07c6fc 100644
--- a/lib/www/server/lib/qc/last-modified.js
+++ b/lib/www/server/lib/qc/last-modified.js
@@ -38,7 +38,98 @@ async function sequenceLastModified (projectId, sequence) {
 	return res;
 }
 
+/** Return or save a hash representing the state of the project.
+ *
+ * The hash is an MD5 digest from the concatenation of all
+ * file paths + file hashes known to Dougal for a given
+ * project.
+ *
+ * The idea is that this should change every time a new
+ * file is imported or when an existing file is changed
+ * or deleted.
+ *
+ * Going only by file timestamp does not work as we may
+ * be importing files with timestamps older than the last
+ * QC run.
+ *
+ * @a projectId The ID of the project to operate on.
+ * @a hash If present, sets info.qc->'hash' to this value,
+ * if absent, return a hash.
+ */
+async function projectHash (projectId, hash) {
+	const client = await setSurvey(projectId);
+
+	if (hash) {
+		const text = `
+			INSERT INTO info (key, value)
+			VALUES ('qc', json_build_object('hash', to_jsonb($1::text)))
+			ON CONFLICT (key)
+			DO UPDATE
+			SET value = jsonb_set(info.value, ARRAY['hash'], to_jsonb($1::text));
+		`;
+
+		const values = [hash];
+		const res = await client.query(text, values);
+		await client.release();
+		return res;
+	} else {
+		const text = `
+			SELECT md5(text) hash FROM (
+				SELECT string_agg(path || E'\t' || hash, E'\n') AS text
+				FROM files
+			) AS t;
+		`;
+
+		const res = ((await client.query(text))?.rows ?? [])[0]?.hash;
+		await client.release();
+		return res;
+	}
+}
+
+/** Return or save a hash representing the state of a sequence.
+ *
+ * Analogous to projectHash() but for a specific sequence.
+ */
+async function sequenceHash (projectId, sequence, hash) {
+	const client = await setSurvey(projectId);
+
+	if (hash) {
+		const text = `
+			UPDATE raw_lines
+			SET
+				meta = jsonb_set(meta, array['lastQCHash'], to_jsonb($2::text))
+			WHERE sequence = $1;
+		`;
+
+		const values = [ sequence, hash ];
+
+		const res = await client.query(text, values);
+		await client.release();
+		return res;
+	} else {
+		const text = `
+			SELECT sequence, md5(string_agg(hash, E'\t')) AS hash
+			FROM (
+				SELECT *
+				FROM raw_lines_files
+				UNION SELECT *
+				FROM final_lines_files
+			) AS t
+			GROUP BY sequence
+			HAVING sequence = $1;
+		`;
+
+		const values = [ sequence ];
+
+		const res = ((await client.query(text, values))?.rows ?? [])[0]?.hash;
+		await client.release();
+		return res;
+	}
+}
+
 module.exports = {
 	projectLastModified,
-	sequenceLastModified
+	sequenceLastModified,
+	projectHash,
+	sequenceHash
 };