diff --git a/src.java/crosby/binary/file/FileBlock.java b/src.java/crosby/binary/file/FileBlock.java
index 6be0616..b8fc198 100644
--- a/src.java/crosby/binary/file/FileBlock.java
+++ b/src.java/crosby/binary/file/FileBlock.java
@@ -28,14 +28,16 @@ public class FileBlock extends FileBlockBase {
 
     public static FileBlock newInstance(String type, ByteString blob,
             ByteString indexdata) {
-        return new FileBlock(type, blob, indexdata);
+      if (blob != null && blob.size() > MAX_BODY_SIZE/2) {
+        System.err.println("Warning: Fileblock has body size too large and may be considered corrupt");
+      }
+      if (indexdata != null && indexdata.size() > MAX_HEADER_SIZE/2) {
+        System.err.println("Warning: Fileblock has indexdata too large and may be considered corrupt");
+      }
+      return new FileBlock(type, blob, indexdata);
     }
 
-    public static FileBlock newInstance(String type, ByteString indexdata) {
-        return new FileBlock(type, null, indexdata);
-    }
-
-    protected void deflateInto(crosby.binary.Fileformat.Blob.Builder blobbuilder) {
+     protected void deflateInto(crosby.binary.Fileformat.Blob.Builder blobbuilder) {
         int size = data.size();
         Deflater deflater = new Deflater();
         deflater.setInput(data.toByteArray());
@@ -77,7 +79,7 @@ public class FileBlock extends FileBlockBase {
             if (flags == CompressFlags.DEFLATE)
                 deflateInto(blobbuilder);
             else
-                assert false : "TODO"; // TODO
+                throw new Error("Compression flag not understood");
         }
         Fileformat.Blob blob = blobbuilder.build();
 
diff --git a/src.java/crosby/binary/file/FileBlockBase.java b/src.java/crosby/binary/file/FileBlockBase.java
index 33644cc..4be4efa 100644
--- a/src.java/crosby/binary/file/FileBlockBase.java
+++ b/src.java/crosby/binary/file/FileBlockBase.java
@@ -13,6 +13,9 @@ import com.google.protobuf.ByteString;
  */
 public class FileBlockBase {
 
+    static final int MAX_HEADER_SIZE = 64*1024;
+    static final int MAX_BODY_SIZE = 8*1024*1024;
+
     protected FileBlockBase(String type, ByteString indexdata) {
         this.type = type;
         this.indexdata = indexdata;
diff --git a/src.java/crosby/binary/file/FileBlockHead.java b/src.java/crosby/binary/file/FileBlockHead.java
index 64204f7..6942b93 100644
--- a/src.java/crosby/binary/file/FileBlockHead.java
+++ b/src.java/crosby/binary/file/FileBlockHead.java
@@ -29,6 +29,10 @@ public class FileBlockHead extends FileBlockReference {
         DataInputStream datinput = new DataInputStream(input);
         int headersize = datinput.readInt();
         // System.out.format("Header size %d %x\n",headersize,headersize);
+        if (headersize > MAX_HEADER_SIZE) {
+          throw new FileFormatException("Unexpectedly long header "+MAX_HEADER_SIZE+ " bytes. Possibly corrupt file.");
+        }
+        
         byte buf[] = new byte[headersize];
         datinput.readFully(buf);
         // System.out.format("Read buffer for header of %d bytes\n",buf.length);
@@ -38,7 +42,10 @@ public class FileBlockHead extends FileBlockReference {
                 .getIndexdata());
 
         fileblock.datasize = header.getDatasize();
-        // data_offset =
+        if (header.getDatasize() > MAX_BODY_SIZE) {
+          throw new FileFormatException("Unexpectedly long body "+MAX_BODY_SIZE+ " bytes. Possibly corrupt file.");
+        }
+        
         fileblock.input = input;
         if (input instanceof FileInputStream)
             fileblock.data_offset = ((FileInputStream) input).getChannel()
diff --git a/src.java/crosby/binary/file/FileBlockPosition.java b/src.java/crosby/binary/file/FileBlockPosition.java
index 51a11a6..f563d16 100644
--- a/src.java/crosby/binary/file/FileBlockPosition.java
+++ b/src.java/crosby/binary/file/FileBlockPosition.java
@@ -27,7 +27,7 @@ public class FileBlockPosition extends FileBlockBase {
 
     /** Parse out and decompress the data part of a fileblock helper function. */
     FileBlock parseData(byte buf[]) throws InvalidProtocolBufferException {
-        FileBlock out = FileBlock.newInstance(type, indexdata);
+        FileBlock out = FileBlock.newInstance(type, null, indexdata);
         Fileformat.Blob blob = Fileformat.Blob.parseFrom(buf);
         if (blob.hasRaw()) {
             out.data = blob.getRaw();
@@ -81,14 +81,12 @@ public class FileBlockPosition extends FileBlockBase {
      * stored.
      */
     public ByteString serialize() {
-        assert false; // TODO
-        return null;
+        throw new Error("TODO");
     }
 
     /** TODO: Parse a serialized representation of this block reference */
     static FileBlockPosition parseFrom(ByteString b) {
-        assert false; // TODO
-        return null;
+      throw new Error("TODO");
     }
 
     protected int datasize;
diff --git a/src.java/crosby/binary/file/FileFormatException.java b/src.java/crosby/binary/file/FileFormatException.java
new file mode 100644
index 0000000..178c958
--- /dev/null
+++ b/src.java/crosby/binary/file/FileFormatException.java
@@ -0,0 +1,16 @@
+package crosby.binary.file;
+
+import java.io.IOException;
+
+public class FileFormatException extends IOException {
+
+  public FileFormatException(String string) {
+    super(string);
+  }
+
+  /**
+   * 
+   */
+  private static final long serialVersionUID = -8128010128748910923L;
+
+}