diff --git a/ExtremelySimpleLogger/FileSink.cs b/ExtremelySimpleLogger/FileSink.cs
index 8253282..46f9085 100644
--- a/ExtremelySimpleLogger/FileSink.cs
+++ b/ExtremelySimpleLogger/FileSink.cs
@@ -51,11 +51,13 @@ namespace ExtremelySimpleLogger {
         /// </summary>
         /// <param name="s">The message to log</param>
         public override void Log(string s) {
-            if (this.reopenOnWrite) {
-                using (var w = this.file.AppendText())
-                    w.WriteLine(s);
-            } else {
-                this.writer.WriteLine(s);
+            lock (this.file) {
+                if (this.reopenOnWrite) {
+                    using (var w = this.file.AppendText())
+                        w.WriteLine(s);
+                } else {
+                    this.writer.WriteLine(s);
+                }
             }
         }
 
@@ -64,8 +66,10 @@ namespace ExtremelySimpleLogger {
         /// </summary>
         public override void Dispose() {
             base.Dispose();
-            if (!this.reopenOnWrite)
-                this.writer.Dispose();
+            lock (this.file) {
+                if (!this.reopenOnWrite)
+                    this.writer.Dispose();
+            }
         }
 
     }
diff --git a/ExtremelySimpleLogger/StringSink.cs b/ExtremelySimpleLogger/StringSink.cs
index 70fad4b..3587824 100644
--- a/ExtremelySimpleLogger/StringSink.cs
+++ b/ExtremelySimpleLogger/StringSink.cs
@@ -12,14 +12,20 @@ namespace ExtremelySimpleLogger {
         /// The string that this sink currently contains.
         /// Can be cleared using <see cref="Clear"/>.
         /// </summary>
-        public string Value => this.builder.ToString();
+        public string Value {
+            get {
+                lock (this.builder)
+                    return this.builder.ToString();
+            }
+        }
 
         /// <summary>
         /// Logs the given message, which has already been formatted using <see cref="Sink.Formatter"/>.
         /// </summary>
         /// <param name="s">The message to log</param>
         public override void Log(string s) {
-            this.builder.AppendLine(s);
+            lock (this.builder)
+                this.builder.AppendLine(s);
         }
 
         /// <summary>
@@ -27,7 +33,8 @@ namespace ExtremelySimpleLogger {
         /// After this call, <see cref="Value"/> will be empty.
         /// </summary>
         public void Clear() {
-            this.builder.Clear();
+            lock (this.builder)
+                this.builder.Clear();
         }
 
     }