Environment variables provide a flexible and secure way to configure CLI applications, enabling developers to avoid hard-coding sensitive data such as database URLs, API tokens, or AWS credentials. In Go, environment variables can be accessed using the `os.Getenv` function, which returns an empty string if the variable doesn't exist, or `os.LookupEnv`, which additionally indicates whether the variable was set. Managing environment variables during development is often streamlined by using a `.env` file to specify key-value pairs, and these can be loaded into the application either via popular packages like `go.env` or through custom loaders that read the file, safely split keys and values, trim any quotes, and set variables using `os.Setenv`. It's important to never commit `.env` files to version control, and variables can be unset when no longer needed. Understanding how to handle environment variables directly gives developers greater insight into application configuration best practices and security.