The Asynchronous Transfer Mode (ATM) is the switching and multiplexing technique chosen by CCITT for the broadband access to the ISDN. The user-network interface offers one physical channel over which connections are multiplexed using short, fixed length packets (called cells). The ATM layer in the network performs relaying functions, with every cell carrying a label used for switching. This report is a tutorial. It aims at providing the reader with background information that is necessary to understand the debate about potential virtues and shortcomings of the ATM. © 1992.