I am wondering why I can't play an H264 AVC .mp4 in Quicktime Player 10. Upon opening Quicktime just says 'Converting'. I mean, I just assumed Quicktime was very good at playing H264 in mp4, and it would just play fine. The video plays fine in VLC understandably, here are the codec details. I was able to play this in Quicktime Player after following http://forums.macrumors.com/showthread.php?p=14725614#post14725614 and running the below command with ffmpeg from http://ffmpegmac.net Code: ffmpeg -i video.mp4 -vcodec copy -acodec copy test.mp4 When playing 'test.mp4' in VLC player it has exactly the same codecs as the screenshot above, which is puzzling me. Why can't Quicktime play this file natively?