<ol> You should check that you're not relying on any applications that will no longer run on Lion. See the article <a href="http://osxdaily.com/2011/07/15/how-to-check-if-applications-are-incompatible-with-mac-os-x-lion/" source="OS X Daily">How to Check if Applications are Incompatible with Mac OS X Lion</a>. Basically, the <i>Rosetta Stone</i> application is not compatible with Lion, so PowerPC-only applications no longer run. If you can't find an update (or there is none announced) for an essential application, then you may have to stick with Snow Leopard for now. If you open the System Profiler and look at <i>Applications</i>, you can see all of the applications that are still PowerPC-only. If you have multiple machines to upgrade, there's a way to do it without downloading (or paying) multiple times. See the article <a href="http://arstechnica.com/apple/guides/2011/07/ask-ars-do-i-have-to-use-the-mac-app-store-to-install-lion.ars" source="Ars Technica" author="Chris Foresman">Do I have to use the Mac App Store to reinstall Lion?</a>. The most important tip is that you have to <iq>copy a disk image buried in the Lion installer app somewhere outside of the Applications folder after you download it.</iq> If you wait until after you've installed, it will already be gone. </ol>