ABSTRACTApplied behaviour analysis has made many contributions to the field of mental handicap in the last two decades, through the development of procedures to treat both adaptive and maladaptive behaviours. At the basis of all its contributions is an insistence that decisions be based upon data.The present paper extends the technology of applied behaviour analysis data collection procedures by describing a computer system developed for collecting and analysing observational data. A brief discussion of the system is followed by examples of four types of research currently undertaken in mental handicap: evaluative, naturalistic, analogue, and experimental research. To conclude, a few weaknesses and strengths of the system are discussed.