Failed to resolve: com.google.firebase:firebase-core:9.0.0

Question

Update Aug 2017

As of version 11.2.0 Firebase and Google Play services dependencies are available via Google's Maven Repo. You no longer need to use the Android SDK manager to import these dependencies.

In your root build.gradle file add the repo:

allprojects {
  repositories {
    // ...
    maven { url "https://maven.google.com" }
  }
}

If you are using gradle 4.0 or higher you can replace maven { url "https://maven.google.com" } with just google().

---

The 9.0.0 version of Firebase was built using Google Play services 9.0 and is now available under the new packaging com.google.firebase:*

See Release Notes for Google Play services 9.0 https://developers.google.com/android/guides/releases#may_2016_-_v90

New versions of packages Google Play Services (rev 30) and Google Repository (rev 26) were just released in the SDK manager so it's likely you just need to update.

---

Downloading Google Play Services and Google Repository

From Android Studio:

1. Click Tools > Android > SDK Manager.
2. Click into the SDK Tools tab.
3. Select and install Google Play Services (rev 30) and Google Repository (rev 26). See the image below.
4. Sync and Build your project.

---

From IntelliJ IDEA:

As of April 2017, the latest versions of Google Play Services and Repository are listed below.

1. Click Tools > Android > SDK Manager.
2. Under the Packages panel, Look for the Extras.
3. Select and install Google Play Services (rev 39) and Google Repository (rev 46). See the image below.
4. Perform a gradle project sync and Build your project.

Go to Android SDK Manager and install the latest version of below two libraries

1. Google Play Services
2. Google Repository

Error:(30, 13) Failed to resolve: com.google.firebase:firebase-auth:9.6.1

If you ever get this error and you are using Android studio 2.2 that comes with firebase component integrated in it which has libraries version 9.6.0 by default and you are adding the latest dependencies like 9.6.1 . You might need to downgrade com.google.firebase:firebase-auth:9.6.1 to com.google.firebase:firebase-auth:9.6.0

Or check the library version of your pre-installed firebase and make sure it is of the same version with the new library you are trying to add or added to your project.

Tried all the above, use the Firebase Assistant! It is the simplest way to solve this. First remove all the dependencies you added to the build.gradle (using the manual method) and then in Android Studio:

Click Tools > Firebase to open the Assistant window.

It really is as easy as that.

Faced myself and seen several times in comments for similar questions - that even after installing "latest" Google Play Services and Google Repository still having the same issue.

The thing is that they may be latest for your current revision of Android SDK Tools, but not that latest your app build requires.

In such case make sure to install latest version of Android SDK Tools first, and probably Android SDK Platform-tools (both under Tools branch). Also please note you may need to go through this several times if you haven't updated for a long time (i.e. install latest Android SDK Tools and Android SDK Platform-tools, then restart Android SDK Manager, then repeat), since the updates seem to be going through some critical mandatory milestones and you cannot install the very latest if you currently have the revision which is pretty "old".

dependencies {
    compile 'com.google.android.gms:play-services-maps:11.8.0'
    compile 'com.google.android.gms:play-services-auth:11.8.0'
    compile 'com.google.android.gms:play-services-ads:11.8.0'
    compile 'com.google.firebase:firebase-storage:11.8.0'

}
apply plugin: 'com.google.gms.google-services'


// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {
    repositories {


        maven { url 'https://maven.fabric.io/public' }

        jcenter()
        google()
    }
    dependencies {
        classpath 'com.android.tools.build:gradle:3.0.0'
        classpath 'com.google.gms:google-services:3.1.1'


        // NOTE: Do not place your application dependencies here; they belong
        // in the individual module build.gradle files
    }
}

allprojects {
    repositories {
        jcenter()
        google()
    }
}

Following are the prerequisites if you want to add firebase to your project.

1. For working with Firebase you should install Android Studio 1.5 or higher.
2. Download the latest Google Play services SDK from through Android SDK Manager.
3. The device should be running Android 2.3 (Gingerbread) or newer, and Google Play services 9.2.0 or newer.

I could only find out all this after hours of struggle.

Source: Adding Firebase to your Android App