We describe a case study of an in-service professional development project with four experienced biology teachers in two upper secondary schools. The objective was to better understand the role of laboratory work (labwork) in science education with a special emphasis on assessment. Research questions include the following: 1. What does the teacher want to achieve with the labwork and how do the students perceive it? 2. How do biology teachers assess labwork? 3. How did the in-service project motivate teachers to change their assessment methods? 4. Are the assessment criteria in the Swedish national syllabus applicable to labwork? Data were collected during a semester-long in-service project. Laboratory instruction sheets were collected and analysed both with respect to intended learning outcome and assessment skills. Questionnaires to teachers and students about the intended learning outcome were carried out after different experiments. Finally, teachers were interviewed concerning the role of labwork and their assessment methods and also their reflections on the results of the in-service project. The teachers realised that their assessment methods require more transparency to become effective. The interpretation of intended learning outcomes of experimental work differs between students and teachers.