Manifest merger failed : uses-sdk:minSdkVersion 10 cannot be smaller than version L declared in library com.android.support:appcompat-v7:21.0.0-rc1

compileSdkVersion 'android-L'

BOOM. Done.

LAST EDIT: As of Android 5.0 release, this is no longer an issue, just target API level 21 directly.

Edit for clarity: Indeed as David_E specified below, this solution only works for L version, if you try to deploy the app on a device below L (ex <=4.4.4) it will complain of OLD_SDK. In order for the app to work pre-L you still need to use the old v20 support lib + app compat + targetVersionSdk and compileVersionSdk

dependencies {

        compile 'com.android.support:appcompat-v7:20.+'
        compile 'com.android.support:support-v4:20.+'
    }

    android {
        compileSdkVersion 20
        buildToolsVersion '20'

        defaultConfig {
            applicationId "com.example.application"
            minSdkVersion 10
            targetSdkVersion 20
            versionCode 1
            versionName "1.0"
        }
}