If You Have an ePUB-based eReader (Nook, iBooks, iBooks Reader for Mac):

There’s a good chance that you can email your ePUB file to your eReading device, if you are using a Nook, iPad, or other eReader that has an email program on it.  

If you’re not, some devices, (Android and iOS-based) will allow you to put a file into a Dropbox, and then open that Dropbox on your device, click the file, and open it in the eReader that way.  (Nooks and iPads worth this way for ePUB files, not MOBIs).  

Otherwise, you’ll have to manually hook your device up to your computer, and then transfer the file by drag-dropping it to the appropriate folder.  We can’t possibly provide you with instructions for all the devices out there, so please Google for the appropriate instructions for your specific device.

If You Have a PC, and Do Not Have An eReader:

You can download the latest Adobe Digital Editions from Adobe, here: https://www.adobe.com/solutions/eBook/digital-editions/download.html

If your question is how to view your ePUB in Adobe Digital Editions, please see the "How to View your ePUB in Adobe Digital Editions" PDF which you can download by clicking here.  (Just select the file you want to download, and when you do, you'll see icons with options on the right—click the down arrow to start downloading the file.)

If you need instructions on installing Adobe Digital Editions on your PC, it should install automatically from the download, once you click the download and "start" that. If that does not happen, please check Adobe for Technical Assistance.  

Readium: you can alternatively use Readium, a browser-based reading app, for the Chrome Browser.  Readium is available here.  

If You Have a Mac:

If you're a Mac user, our highest and best recommendation is that you download and install iBooks Reader for Mac, from the iTunes Store.  Sorry that we don't have a link for that—you should tell Apple not to be so difficult!

Once you've done that, you should be able to open your .ePUB file, from within Finder, using iBooks Reader for Mac.

If you have an iPad:

As discussed above, you should be able to open your ePUB on your iPad using iBooks.  You can do that by emailing the ePUB file to yourself, or by opening the email that we sent you, downloading the ePUB file to your iPad, and then "open with" iBooks. You can side-load the .ePUB file to your iBooks' directory, or you can put it in an iPad-linked Dropbox, and "open with" iBooks.  There are numerous ways to get your ePUB onto your iPad. The same is not true for your MOBI file, so please do not try to directly open the MOBI file on your iPad.  There are special instructions for that.  

If your question is for transferring your Amazon Mobi file to your iPad or iPhone, How to Put your MOBI file onto your iPad for Review.