This chapter is a dense review of basic concepts related to distributed algorithms. We review computer networks with focus on network layers as a distributed algorithm uses network communications to send/receive messages with a peer node. We then describe basic message passing operations in a distributed system. The execution modes of a distributed algorithm may vary, it can be synchronous or asynchronous, a single initiator or a multiple initiators as described. A distributed algorithm may be represented and modelled by various visual aids such as space-time diagrams or finite state machines as we review. Lastly, a distributed algorithm should be analysed to assess its performance and its correctness should be proven as we describe.