NOTE: This post contains memtest 4.22 which supports OS X 10.6.8 and below. The newest version is memtest 4.23, at http://www.memtestosx.org, which supports OS X 10.7 and above. Memtest is an essential tool for testing your RAM. If you are experiencing kernel panics or freezes, your RAM should be one of your first suspects. Or if you've just upgraded your Mac's RAM, and want to ensure that it is all good before you sell the old RAM. Or you merely want to make sure your RAM is good, so that in the future you can eliminate it as the source of the problem. Whatever your situation is, if you need to test your RAM, Memtest OS X is the best tool for the job for your Mac. Yes, there's Memtest86+ - I always use that for all the PCs I own. However, there's a bug in Memtest86+ where it could give false positives on EFI machines (which all Intel Macs are). Memtest, by the way, is open source. While their website requires you to pay a fee to download it, I am providing it here for free. The source code is included to comply with the GPL2 license. To start the program, simply start it in the Terminal. It is suggested, however, that you use it in Single-User Mode for best results. To boot into Single-User Mode, hold command-S on boot. Then type those commands: Code: /sbin/fsck -fy /sbin/mount -uw / The first command is a quick checkup of your HD. Not strictly necessary, but it only takes a few seconds, and is generally a good idea. Once the HD is mounted, cd (change directory) to where the memtest program is located. For example, if you put memtest in your Applications folder, the path would be: Code: cd /Applications Or if you put it in the Utilities folder, it would be Code: cd /Applications/Utilities And so on. You get the idea. Then you can run memtest with a simple "./memtest" command. However, there's stuff you can customize. The first is how much memory you want to test. I'd say all, so just put "all" after "./memtest". Then comes how many times you want to run the test. 3 to 5 are good numbers, so just put that number after "all". Then you can decide whether you want the results logged or not with a -L. There's more, but those are the basics, and all you need to know. So a simple 3-round memtest test of all available memory would be: Code: ./memtest all 3 That's it.