# Google authentication
Looks for credentials in the following places, preferring the first location found:
- A JSON file whose path is specified by the GOOGLE_APPLICATION_CREDENTIALS environment variable.
- A JSON file in a location known to the gcloud command-line tool using `gcloud auth application-default login`.
- On Google Compute Engine, it fetches credentials from the metadata server.
## Local development
Don't confuse `gcloud auth login` with `gcloud auth application-default login` for local development,
since the first authorize only `gcloud` tool to access the Cloud Platform.
The latter obtains user access credentials via a web flow and puts them in the well-known location for Application
Default Credentials (ADC).
This command is useful when you are developing code that would normally use a service account but need to run the code
in a local development environment where it's easier to provide user credentials.
So to work for local development you need to use `gcloud auth application-default login`.