This unit provides an understanding of basic principles on which modern distributed systems are based including models, algorithms and design. In the opening session, learners are introduced the conceptual overview of the subject as well as characteristics of distributed system. Also, the challenges during the process of analyzing and developing distributed systems are addressed. In the next part, the details of communication, middleware, naming service and security are covered to consolidate the theory how distributed systems work. Finally, specific topics such as synchronization, replication, failure-tolerance and security are presented with detailed case studies.