The problem based learning (PBL) curriculum at Manchester emphasizes small-group work. This is supported through group assessment where students assess key aspects of their group's function. In the study described here the authors evaluated students' perceptions of both PBL group work and what a group assessment needs to assess. They aimed to produce a description of the cognitive and motivational influences on group process and unpack the ways they contribute to a successful PBL group so that the kinds of skills an effective assessment should assess could be identified. Focus groups and a questionnaire were used to generate the data. The focus group results indicate that students support PBL group work as a method of learning, and that those groups that work cooperatively are perceived as facilitating the most motivating learning environment. The students supported the assessment being summative and felt that it could be simplified to measure: behavioural skills contributing to maximizing motivation of the group process; and cognitive skills relating to the content of the group discussion. The questionnaire results also supported the use of a summative assessment of small-group work that evaluates the domains of group process and the content of the group discussion.