> ## Documentation Index > Fetch the complete documentation index at: https://circle-devdocs-test-ai-codegen-component.mintlify.site/llms.txt > Use this file to discover all available pages before exploring further. # Getting Started > Connect your applications to Circle to use USDC for payments, payouts, and transfers. Circle APIs are a set of functions and procedures that allow you to use USDC for payments, payouts, and transfers. They enable your applications to connect to banks and digital wallets and enable global payments via USDC. By integrating with our APIs to automate minting and redemption, you can provide stablecoin access directly to your users through your own app or website.\_ To ensure success, please note the following: * API requests without authentication will fail. * All API requests must be made over HTTPS. * Keep API keys secure at all times. Ensure your API key is always stored securely and never share it or record it in a medium that might be publicly accessible such as client-side code, public repositories, etc. ## API key authentication Set the API key in the `Authorization` header of the request sent from your backend server using the format `Bearer secret-key-value`. ```text API Authentication theme={null} Authorization: Bearer YOUR_API_KEY ``` ## API key in Sandbox By using the Circle [sandbox environment](/circle-mint/circle-apis-production-sandbox-environments), you can quickly familiarize yourself with the Circle API request and response formats and build prototypes. The API key you use in sandbox will be different than the one you use in production. If you don't yet have an API key in the sandbox environment, take a few moments to [create a sandbox account](https://app-sandbox.circle.com). After you create your sandbox account and generate your API key, follow the links below to continue exploring Circle APIs.