I just set up Time Machine, and in order to make my backups efficient, I'm trying to think what kind of stuff to exclude. I want a full, working backup, so that if I needed to I could have a complete, bootable restore in less than an hour. With that in mind, here's what I excluded so far: 1) My 2nd external hard drive. It's filled with videos and wouldn't fit anyway. I figure if that drive goes down I'll recover what I can from it and re-rip the rest. 2) My downloads folder. I just download so much crap here that I usually throw away instantly or else move. Plus the downloads folder usually has 2 copies of everything: compressed and uncompressed. Even worse, if the download is a .tar.gz, it has 3 copies of the file. 3) My VMWare images folder. Time machine will back this sucker up every hour that I'm using one of the virtual machines (due to the fact that the guest OS is storing virtual memory in the image file), which would completely kill my ability to go "back in time" since it would fill up my drive very quickly. 4) Anything that's already version controlled elsewhere. I have all my code in a subversion repository on another server, so I don't see a point in backing that up. The backup would just be cluttered anyway. Anything else? I like the idea of making really thorough backups, so I don't want to be stingy with drive space, I'm really just focusing on providing as much "history" as possible in backup.