If you use a Mac and Photoshop Elements and plan to upgrade to Yosemite (OS X 10.10) right away, just a reminder that in all recent versions of OS X, it has been critically important to delete all preferences before running PSE.
If you have once run PSE it is necessary to also delete the saved application states.
So you would need to go to your user library and delete:
Adobe Photoshop Elements < version > Paths
Adobe Photoshop Elements < version > Settings
com.adobe.Elements Organizer. < version >.plist
and any lockfiles with the same names, then (if you’ve used Elements in Yosemite) go to Saved Application States (at the same level as preferences) and delete any for PSE, then repair permissions and launch PSE again.
To see the User library, click the Go menu in the Finder and hold down the Option key and it will appear below the little house for your user account.
I’m not using Yosemite myself yet, but I would not be surprised if some of the older versions of PSE that work fine in 10.9 no longer work in 10.10, so I would suggest that if Elements is important to you, be sure you have a good clone of your system somewhere so that you can revert it if necessary.
EDIT 9/19/2014: Some people are having a lot of trouble with Elements 12 in Yosemite, especially the tools in the Enhance section and layer behavior, so if you are using Elements 12 and don’t wish to upgrade to PSE 13, you might want to stop and think before going to 10.10 if PSE is important to your work.