With the launch of OS X 10.10.3 Yosemite, Apple released its much-anticipated Photos app, which was first announced during its Worldwide Developers Conference in 2014. The Photos for OS X app took several months to complete because it was designed from the ground up to work with OS X Yosemite and integrate with both the Photos for iOS app and iCloud Photo Library.
Because Photos for OS X replaces both of Apple's existing photo apps, Aperture and iPhoto, you're going to need to migrate your Aperture and iPhoto libraries into the new Photos app if you want to be able to use Apple's latest and greatest photo editing tools with your images.
One quick note: Before you migrate all of your existing photos into Photos for OS X, make sure you have enough iCloud storage space if you want to take advantage of iCloud Photo Library, which syncs your images across al of your devices. With iCloud Photo Library turned on in the Photos app, all of your images will be uploaded to the cloud, which does eat up your iCloud storage space. You can use Photos without iCloud Photo Library -- just make sure you turn it off in the Preferences menu.
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Article Link: How to Migrate Pictures from iPhoto or Aperture to Photos for OS X