Application security remains a long-term and high-stakes problem for most projects that interact with external users. Python's type system is already widely used for readability, refactoring, and bug detection — this talk will demonstrate how types can also be leveraged to make your project systematically more secure. We'll investigate (1) how static type checkers like Pyre or MyPy can be extended with simple library modifications to catch vulnerable patterns, and (2) how deeper type-based static analysis can reliably flag remaining use cases to security engineers. As an example, I'll focus on a basic security problem and how you might use both tools in combination, drawing from our experience deploying these methods to build more secure applications at Facebook and Instagram.