This chapter presents a development of Perlmutter’s (1978) unaccusative hypothesis. It argues that the verbal domain should be considered to comprise an ordered series of functional heads here termed the VISCO hierarchy, and that this approach permits an improvement understanding of split intransitive behaviours. The history of research into unaccusativity and split intransitivity is considered, with the strengths and weaknesses of the proposals made by Perlmutter (1978), Burzio (1986), Levin & Rappaport Hovav (1995) and others discussed and compared to the VISCO approach. The VISCO hierarchy is also compared to the hierarchy proposed in Ramchand (2008) and discussed in relation to the work of Sorace (2000). Issues such as difficulties in classifying unergatives/unaccusatives within a single language, apparent variation between languages, and the problem of syntax–semantics linking are all considered.