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.

Turning off the Instant run removed my error for Androdi Studio 2017.03.03 v2.3