--- a/build.gradle
+++ b/build.gradle
@@ -45,19 +45,22 @@ buildscript {
             }
         }
         // For in tree plugins.
         maven {
             url "file://${gradle.mozconfig.topsrcdir}/mobile/android/gradle/m2repo"
         }
     }
 
+    ext.kotlin_version = '1.1.51'
+
     dependencies {
         classpath 'com.android.tools.build:gradle:3.0.1'
         classpath 'com.getkeepsafe.dexcount:dexcount-gradle-plugin:0.8.2'
+        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
     }
 }
 
 if ('multi' == System.env.AB_CD) {
     // Multi-l10n builds set `AB_CD=multi`, which isn't a valid locale.  This
     // causes the
     //
     // |mach build| > |mach gradle| > |make gradle-targets| > AndroidManifest.xml > strings.xml > multi/brand.dtd

--- a/mobile/android/geckoview/build.gradle
+++ b/mobile/android/geckoview/build.gradle
@@ -1,11 +1,12 @@
 buildDir "${topobjdir}/gradle/build/mobile/android/geckoview"
 
 apply plugin: 'com.android.library'
+apply plugin: 'kotlin-android'
 
 apply from: "${topsrcdir}/mobile/android/gradle/product_flavors.gradle"
 
 // This converts MOZ_APP_VERSION into an integer
 // version code.
 //
 // We take something like 58.1.2a1 and come out with 5800102
 // This gives us 3 digits for the major number, and 2 digits
@@ -146,20 +147,22 @@ android {
         }
     }
 }
 
 dependencies {
     implementation "com.android.support:support-v4:${mozconfig.substs.ANDROID_SUPPORT_LIBRARY_VERSION}"
     implementation "com.android.support:palette-v7:${mozconfig.substs.ANDROID_SUPPORT_LIBRARY_VERSION}"
 
+    testImplementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
     testImplementation 'junit:junit:4.12'
     testImplementation 'org.robolectric:robolectric:3.5.1'
     testImplementation 'org.mockito:mockito-core:1.10.19'
 
+    androidTestImplementation "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version"
     androidTestImplementation 'com.android.support.test:runner:0.5'
     androidTestImplementation 'com.android.support.test:rules:0.5'
     androidTestImplementation 'com.android.support.test.espresso:espresso-core:2.2.2'
     androidTestImplementation "com.android.support:support-annotations:${mozconfig.substs.ANDROID_SUPPORT_LIBRARY_VERSION}"
 }
 
 apply from: "${topsrcdir}/mobile/android/gradle/with_gecko_binaries.gradle"
 
@@ -215,17 +218,17 @@ android.libraryVariants.all { variant ->
     }
 }
 
 android.libraryVariants.all { variant ->
     configureLibraryVariantWithJNIWrappers(variant, "Generated")
 }
 
 apply plugin: 'maven'
- 
+
 uploadArchives {
     repositories.mavenDeployer {
         pom.groupId = 'org.mozilla'
         pom.artifactId = "geckoview-${mozconfig.substs.MOZ_UPDATE_CHANNEL}-${mozconfig.substs.ANDROID_CPU_ARCH}"
         pom.version = mozconfig.substs.MOZ_APP_VERSION
         pom.project {
             licenses {
                 license {
@@ -259,13 +262,25 @@ afterEvaluate {
 
     artifacts {
         // Instead of default (release) configuration, publish one with Gecko binaries.
         archives bundleOfficialWithGeckoBinariesNoMinApiRelease
         // Javadoc and sources for developer ergononomics.
         archives javadocJarOfficialWithGeckoBinariesNoMinApiRelease
         archives sourcesJarOfficialWithGeckoBinariesNoMinApiRelease
     }
+
+    // For now, ensure Kotlin is only used in tests.
+    android.sourceSets.all { sourceSet ->
+        if (sourceSet.name.startsWith('test') || sourceSet.name.startsWith('androidTest')) {
+            return
+        }
+        (sourceSet.java.srcDirs + sourceSet.kotlin.srcDirs).each {
+            if (!fileTree(it, { include '**/*.kt' }).empty) {
+                throw new GradleException("Kotlin used in non-test directory ${it.path}")
+            }
+        }
+    }
 }
 
 // Bug 1353055 - Strip 'vars' debugging information to agree with moz.build.
 apply from: "${topsrcdir}/mobile/android/gradle/debug_level.gradle"
 android.libraryVariants.all configureVariantDebugLevel