Simultaneous interpreting, an apparently unnatural form of human communicative behaviour, can be regarded as the cornerstone of interpreting as a profession and as a field of academic study. Though it is typically associated and even equated with the highly professionalized practice of conference interpreting, simultaneous interpreting is, strictly speaking, one of two basic modes in which the translational activity of interpreting can be carried out. Based on this understanding, this chapter describes simultaneous interpreting with special regard to its historical development, the models drawn up to capture its underlying neurolinguistic and cognitive processes, and some of the major issues investigated in past and present research.