Firebase messaging, where to get Server Key?

Solution:

1. Click the Settings (Cog wheel) icon next to your project name at the top of the new Firebase Console, as per screenshot below:
2. Click Project settings.
3. Click on the Cloud Messaging tab.
4. The key is right under Server Key.

Take this one: (First answer is right - this one is just to show how the page looks like!) https://support.clevertap.com/docs/android/find-your-fcm-sender-id-and-fcm-server-api-key.html#

UPDATE: As of April 2017 this method does not work. The api_key in google-services.json is only the legacy key. The selected answer is correct.

Deprecated way:

In your project there is a JSON file called google-services.json

Open that file and search for

"api_key": [
        {
          "current_key": "********"
        }

those *** are your server api key

If you dont have that file then you can download it from your app details in firebase account