TutorWhile assessment and its effects on learning such as "washback" have been the subject of much debate (e.g. Alderson & Wall, 1993; Bachman, 1995: 356; Alderson & Hamp Lyons, 1996; Shohamy "et al", 1996; Andrews "et al", 2002), assessment as a tool for developmental, not merely evaluative, purposes has probably not received sufficient attention (e.g. Dickinson, 1996: 101; Gardner, 1999). Of interest to this study is a project by Dickinson (1996: 106) in which participants (tutors and learners) are shown to have benefited from a scheme of collaborative assessment. This paper reports on a study investigating the impact of collaborative assessment on language development and learning. It involved two groups of second-year students at the Hong Kong Polytechnic University enrolled on an English in the Workplace (EIW) course in 2002 and 2003. Two questions were explored: (1) What benefits and difficulties exist in collaborative assessment? (2) What effects does collaborative assessment have on learning? The paper begins with an account of the procedures adopted for the study, followed by its findings and a discussion of the findings. Students cited the attitude of the tutor, unfamiliarity with collaborative assessment and time constraints as the main difficulties encountered but noted perceptible gain in areas of language, meta-language and effect. There is also evidence which points to a shift of participants' roles from "passive learner" to "active participant" as well as change in the use of learning strategies. (Contains 2 figures.)