The purpose of this paper is to show that the distinctive nature of information systems research, being concerned conjointly with the study of artefacts and human behaviour, requires a meta-understanding of theory that draws on work from the philosophy of science (both natural and social) in addition to work on the sciences of the artificial. Some of the misunderstandings surrounding the concept of positivism also need to be exposed and removed, so that full value can be gained from readings in the philosophy of science, which underpins notions of theorising. The outcome is that five interrelated forms of theory can be distinguished, all of which are needed for information systems research. Acknowledging the contribution that each type of theory can make will allow work in information systems to proceed in a more integrated manner.