This chapter introduces the macroscopic views on distributed systems’ components and their inter-relations. The importance of the architecture for understanding, designing, implementing, and maintaining distributed systems is presented first. Then the currently used architectures and their derivatives are analyzed. The presentation refers to the client-server (with details about Multi-tiered, REST, Remote Evaluation, and Code-on-Demand architectures), hierarchical (with insights in the protocol oriented Grid architecture), service-oriented architectures including OGSA (Open Grid Service Architecture), cloud, cluster, and peer-to-peer (with its versions: hierarchical, decentralized, distributed, and event-based integration architectures). Due to the relation between architecture and application categories supported, the chapter’s structure is similar to that of Chapter 1. Nevertheless, the focus is different. In the current chapter, for each architecture the model, advantages, disadvantages and areas of applicability are presented. Also the chapter includes concrete cases of use (namely actual distributed systems and platforms), and clarifies the relation between the architecture and the enabling technology used in its instantiation. Finally, Chapter 2 frames the discussion in the other chapters, which refer to specific components and services for large scale distributed systems.