Security APIs are designed to enable the storage and processing of confidential data without that data becoming known to individuals who are not permitted to obtain it, and are central to the operation of Automated Teller Machines (ATM) networks, Electronic Point of Sale (EPOS) terminals, set-top boxes for subscription-based TV, pre-payment utility meters, and electronic ticketing for an increasing number of public transport systems (e.g., Oyster in London). However, since the early 2000s, it has become clear that many of the security APIs in widespread use contain subtle flaws which allow malicious individuals to subvert the security restrictions and obtain confidential data that should be protected. In this thesis, we attempt to address this problem by presenting a type system in which specific security properties are guaranteed to be enforced by security APIs that are well-typed. Since type-checking is a form of static analysis, it does not suffer from the scalability issues associated with approaches that simulate interactions between a security API and one or more malicious individuals. We also show how our type system can be used to model an existing security API and provide the same guarantees of security that the API authors proved it upholds. This result follows directly from producing a well-typed implementation of the API, and demonstrates how our type system provides security guarantees without requiring additional API-specific proofs.