Make Photos for OS X shine — add photos by importing your iPhoto library.
Photos for OS X makes it easy for you to import your iPhoto Library into the app. If you just have a single iPhoto Library, Photos will do the work automatically for you. If you have multiple libraries, you'll have to choose which one you want to import. Here's how.
If you have a single iPhoto Library
If you're upgrading to Photos for OS X from iPhoto and you've only ever had a single iPhoto library on your Mac, your upgrade path is easy: After you open the app for the first time, Photos will automatically import all your iPhoto images into the app.
Your old iPhoto library will still remain if you need to use iPhoto for whatever reason, but changes made to those older images won't automatically sync to your new Photos library. If you don't need your old iPhoto library, you can toss it in the trash — your images are now safely in Photos (and, if you've turned on iCloud Photo Library, in iCloud as well).
If you're working with multiple libraries
Photos for OS X is only compatible with one primary library per Mac: This means that you can't combine multiple old iPhoto or Aperture libraries into one master library. (Aperture does have this capability, if you need to merge some libraries; you can follow our how-to guide for more information on that front.)
As such, if you have multiple libraries on your machine, Photos will ask you to select which library you'd like to import when you first launch the app. Once you've selected the library you want to use, Photos will prep and import those images.
You can still upgrade your other older libraries to separate Photos libraries; you'll just have to individually import each one by option-clicking the Photos icon when starting the app. This brings up the "Choose Library" screen, where you can select a current Photos library or an older iPhoto or Aperture library to import.
These other Photos libraries function identically to each other, with one exception: Only one library can be synced with iCloud Photo Library at a time. Your others will be locally (or externally, if you have them on a hard drive) siloed from iCloud's sync service.