I've been using Fission for professionally archiving and re-posting streaming internet radio programming since it's release. Despite claims made below, all fully-featured sound file editors convert MP3s to PCM audio before editing and then must re-encode when editing is done. This results in a serious quality hit, which might be an acceptable trade off if you must process and change the signal- however, if all you want to do is edit MP3s with no change to the signal and no loss in quality, Fission is the only option. It's also a great option. It's been fast and reliable, Rogue Amoeba keeps refining it, support is top-notch and there has not yet been any charge for any upgrade. I think $30 is more than fair for this, however I only paid $18 because I am a user of Rogue Amoeba's Audio Hijack Pro. The only feature missing for me is to take multiple MP3s and simply concatenate them. That's the only reason I still have MP3 Trimmer on my computer. If you need to do full featured audio editing from a variety of formats using plug-ins, etc., there are many options for the Mac, both commercial and shareware. If you need lossless, easy MP3 editing, you need Fission.