Ruby applications are becoming more complex and unwieldy (think MonoRails). An increasing number of us are trying to address this by separating our apps into gems without knowing best practices on how to write APIs or how to manage dependencies and versioning. The discussion we often have around good APIs should not stop with Ruby OO design, but should also consider the public APIs of gems. This talk is intended for gem authors and users. It will share though many practical examples using the Ruby driver to MongoDB 1) Principles of good gem API design, 2) How to make your gem API optimal for users and how to best use the API of a gem, 3) Guidelines for introducing and anticipating gem API changes and managing dependencies.