This dissertation describes architecture optimisation, a novel optimisation technique. Architecture optimisation improves the performance of software components or applications by modifying the way in which they communicate with other components, or with the operating system. This is a significantly different focus to traditional compiler optimisations, which typically operate on a single application and do not attempt to change the way it interacts with the rest of the system. To perform an architecture optimisation, the author of a programming interface writes a small, domain-specific optimisation specification which describes both the conditions necessary for the architecture optimisation to be valid, and the way in which such an optimisation should be performed. This specification is then used as input to an architecture optimiser, which applies the optimisation to a particular application. Architecture optimisation does not require application source code, effectively decoupling optimisation from compilation. To demonstrate its usefulness, an implementation of architecture optimisation, named Currawong, is described. Currawong is a complete architecture optimiser, supporting two languages (Java and C) and two completely different software platforms (the Android smartphone operating system, and CAmkES, a research-focused component-based system). Currawong is applied to several optimisable applications on both platforms, and achieves significant performance improvements. The two major contributions of the work are a concise specification language for architecture optimisations, and early proof that the technique is useful for real-world applications, in the form of benchmark results demonstrating significant (up to 2x) performance improvements.