What types of authentication are required for Google API Services?

Google API Services require various authentication methods to ensure secure access, depending on the type of API and data sensitivity. Basic forms of authentication include API keys for simple access, OAuth 2.0 for secure user authentication, and service accounts for a server-to-server interaction. A typical scenario where OAuth 2.0 might be used is when the application needs either to read or write user data. For most other, less sensitive needs, API keys are sufficient. Google has ample documentation on the different authentication methods, thus enabling developers to choose which one will work best for their application.