When your iPhoto library starts getting very large, or you transfer it from one computer to another (and these are just examples, there are other potential causes) you may need to rebuild your thumbnail cache. You can otherwise be prone to uncharacteristic sluggishness while scrolling, weird thumbnail duplication, or they might just be missing altogether.

It’s easily accomplished by first making sure iPhoto is not open, and holding down the Command and Option/Alt keys while you click on the iPhoto icon on the Dock at the bottom of the screen. You’ll then be prompted with the same options shown in the window below. Select the options you think most appropriate and click ‘Rebuild’. See, people? This is why multiple-choice tests exist… So we can learn to apply tedious logic to computer menus.

Click the iPhoto icon in the dock while holding down Command and Option/Alt to bring up these options

As usual, if you have any specific questions about which options may be best for a particular situation, feel free to leave a comment.