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Downloading songs for offline use is only possible if you have a Spotify Premium subscription. You also need to be connected to Wi-Fi in order to download your Spotify music—you can't download songs over cellular data. Tap the Spotify app icon, which resembles three black, horizontal bars on a green background. This will open your Spotify Home page if you're logged in. If you aren't logged in, tap LOG IN, then enter your Spotify account's email address and password. It's a tab in the bottom-right corner of the screen. This tab is either at the top of the screen (iPhone) or in the middle of the page (Android). Doing so opens a list of your Spotify playlists.  You can also select Albums on this page. If you want to download every song in your library on Android, tap Songs here instead, then skip the next step. On the "Playlists" page, tap the playlist that contains the songs you want to download. If you opened the Albums page instead, select an album. It's near the top-right corner of the screen. The switch will turn green {"smallUrl":"https:\/\/www.wikihow.com\/images\/1\/15\/Iphoneswitchonicon1.png","bigUrl":"\/images\/thumb\/1\/15\/Iphoneswitchonicon1.png\/46px-Iphoneswitchonicon1.png","smallWidth":460,"smallHeight":300,"bigWidth":"46","bigHeight":"30","licensing":"<div class=\"mw-parser-output\"><p>I edited this screenshot of an iPhone\n<\/p><p>License: <a rel=\"nofollow\" class=\"external text\" href=\"http:\/\/en.wikipedia.org\/wiki\/Fair_use\">Fair Use<\/a><br>\n<\/p><\/div>"} to signify that the songs are downloading onto the Spotify mobile app. When a song finishes downloading, it will have a downward-facing arrow icon to the right of it. Whenever you're not connected to the Internet, you can open Spotify, tap the Your Library tab, go to the location of the music you want to listen to, and tap a song to begin listening.
Make sure you have a Spotify Premium subscription. Open Spotify. Tap Your Library. Tap Playlists. Select a playlist. Tap the grey "Download"  switch. Listen to Spotify music offline.