At first glance, archaea and bacteria look alike; however, the composition of the archaeal cell envelope is fundamentally different from the bacterial cell envelope. With just one exception, all archaea characterized to date have only a single membrane and most are covered by a paracrystalline protein layer. This Review discusses our current knowledge of the composition of the archaeal cell surface. We describe the wide range of cell wall polymers, O- and N-glycosylated extracellular proteins and other cell surface structures that archaea use to interact with their environment.