Session 'app': Error Installing APK
Edit:
In newer Android Studio versions you can re-sync the project using this button:
For older versions:
Open Gradle window (on the right side in Android Studio) and click on the refresh button.
However it is not a 100% sure fix.
Solutions for other cases:
Open terminal window and type "adb kill-server", then type "adb start-server". Usually after a few hours of inactivity, adb used to disconnect the device. (If you don't have the sdk/platform-tools in the PATH environment variable, then you should open a terminal in that folder)
One tip if these solutions don't help you:
If you open the Event Log window in the right bottom corner of Android Studio, you can see a detailed error message.
Other edge case
If you see this error: INSTALL_FAILED_INVALID_APK:... signatures are inconsistent.
Then unfortunately a gradle refresh isn't enough, you have to go to Build -> Clean Project and then Run again.
Issue with Android emulator
If you want to deploy the APK to an Android Emulator and you see the "Error installing APK" message, your emulator may be frozen and need restart.
How to fix 'Session 'app': Install failed.'
try with Invalid caches and restart
File > invalidate caches/restart > invalidate and restart
Related Topics
Disable Icon Colorstatelist in Navigationview
Android Edittext Listener for Cursor Position Change
Android, How to Exec a SQL File in Sqlitedatabase
Changing Values from Cursor Using Simplecursoradapter
How to Add a Custom Item to a Navigationview with a Menu Layout
Creating a Seterror() for The Spinner
Putting Screen Densities into The Correct Bucket
Does Android Studio Have a Hierarchy Viewer or Layout Inspector
Change Edittext Password Mask Character to Asterisk (*)
Rejecting Incoming Call in Android
Android Service to Check Internet Connectivity
Register Receiver in a Service
Action Bar Back Button Not Working
How to Center The Camera So That Marker Is at The Bottom of Screen? (Google Map API V2 Android)