The central role of "platform" products and services in mediating the activities of disaggregated "clusters" or "ecosystems" of firms has been widely recognized. But platforms and the systems in which they are embedded are very diverse. In particular, platforms may exist within firms as product lines, across firms as multi-product systems, and in the form of multi-sided markets. In this paper we argue that there is a fundamental unity in the architecture of platforms. Platform architectures are modularizations of complex systems in which certain components (the platform itself) remain stable, while others (the complements) are encouraged to vary in crosssection or over time. Among the most stable elements in a platform architecture are the modular interfaces that mediate between the platform and its complements. These interfaces are even more stable than the interior core of the platform, thus control over the interfaces amounts to control over the platform and its evolution. We describe three ways of representing platform architectures: network graphs, design structure matrices and layer maps. We conclude by addressing a number of fundamental strategic questions suggested by a unified view of platforms.