If you are watching a DVD on the Mac, you may find that you are unable to capture a screenshot while the player is running or open. I guess they are worried about piracy or copyright control or something.

If this is the case, there is way to capture the screenshot and that is to use the screencapture command available in the terminal. To use this open a terminal window, this application can be found in Applications > Utilities. Once open, you can type "screencapture -S ~/Desktop/image1.png" (without the quotes) to capture a screenshot and save the result into a file called image1.png in the Desktop directory. You can change the directory and file name to whatever you want. In addition, instead of the -S, you can use a -i flag and you'll then find your mouse altered so that you can highlight a section of the screen and then only that section of the screen will be captured in the screen shot.