Traditional assessment is based on perceiving and measuring knowledge possession and practical performance. This kind of assessment provides limited information about the capability of the learner to develop as a professional and to learn at work. In a typical skill test situation the teachers estimate how well the students know knowledge content and the work supervisors appraise how they perform in practice. However, reflective and social knowing are weakly assessed because of the problems with tacit and potential knowledge. Context-based assessment requires that situational and contextual factors of knowing, and the social, reflective, cognitive processes of learning are considered carefully. The article reports a construction of a set of assessment criteria and piloting it in a field of vocational education. Carefully defined observation units and optimal compiled criteria are the key factors for understanding and improving the assessment practices. (Contains 1 table and 3 figures.)