diff --git a/ocat.c b/ocat.c
index 108ab0f..5a4e7ef 100644
--- a/ocat.c
+++ b/ocat.c
@@ -136,6 +136,12 @@ void usage(char *prog)
 	printf("  --last		-L     JSON object with last users\n");
 	printf("  --killdata                   requires -u and -d\n");
 	printf("  --storage		-S     storage dir (./store)\n");
+	printf("\n");
+	printf("Options override these environment variables:\n");
+	printf("   $OCAT_USERNAME\n");
+	printf("   $OCAT_DEVICE\n");
+	printf("   $OCAT_FORMAT\n");
+	printf("   $OCAT_STORAGEDIR\n");
 
 	exit(1);
 }
diff --git a/ot-recorder.c b/ot-recorder.c
index 2ae287e..d52aa80 100644
--- a/ot-recorder.c
+++ b/ot-recorder.c
@@ -665,6 +665,14 @@ void usage(char *prog)
 	printf("  --http-port <port>	-A     HTTP port (8083)\n");
 	printf("  --doc-root <directory>       document root (./wdocs)\n");
 #endif
+	printf("\n");
+	printf("Options override these environment variables:\n");
+	printf("  $OTR_HOST		MQTT hostname\n");
+	printf("  $OTR_PORT		MQTT port\n");
+	printf("  $OTR_STORAGEDIR\n");
+	printf("  $OTR_USER\n");
+	printf("  $OTR_PASS\n");
+	printf("  $OTR_CAFILE		PEM CA certificate chain\n");
 
 	exit(1);
 }