We examined the use of functional analysis methodologies to identify the environmental determinants of challenging behaviour for two students with severe disabilities. With one student the functional analysis was conducted in an outpatient clinic which was removed from the school setting. The functional analysis was conducted in the classroom setting with the other student. These assessments produced clear hypotheses regarding the controlling contingencies for challenging behaviour with both students. Individualised support plans were then developed and were successfully implemented by classroom personnel. The results of this study are discussed in terms of using a flexible approach to functional assessment in schools based on the individual characteristics of the student and the classroom context.