The overarching interest in this project is to gain knowledge about what happens in the meeting between group work and assessment in pedagogical practice. Grounded theory methodology, was used as methodology and data have been gathered by focus group interviews with 11 teachers teaching students aged 11 to 16 years. The results show that teachers primarily used informal approaches, a general language and the descriptions about what they assessed and how assessment was carried out. This article elucidates some of the teachers’ problems concerning assessment in group work and some pedagogical implications based on empirical findings. Background: The overarching interest in this project is to gain knowledge about what happens in the meeting between group work and assessment in pedagogical practice. There seems to be a tension between the demand for individual assessment of students’ knowledge and abilities and the demand to teach students collaboration abilities through group work. A previous study concerning teachers’ management of group work as a classroom activity (Hammar Chiriac & Forslund Frykedal, 2011) reveals that assessment is a highly relevant factor. In addition, teachers seem to experience difficulties and acknowledge some challenges and problems as regards assessing students working in a group. Purpose: The main purpose of this study was to obtain not only increased knowledge of how teachers assess individual learning occurring in group work, but also explore how teachers manage the emerging challenges. An additional purpose was, with support from previous research within the area, to provide a means of handling these challenges. Sample: Data were gathered by means of three focus group interviews. The informants were 11 teachers. They taught students aged 11 to 16 years in compulsory education in Sweden during the spring of 2009. Design and method: Grounded theory methodology, together with the theoretical perspective of symbolic interactionism, was used to increase comprehension of the teachers’ problems when assessing learning outcome in group work. The transcribed discussions in the focus groups were analysed by means of theoretical sampling, and a number of categories emerged. Results: The results show that teachers primarily used informal approaches when they assessed students’ knowledge and abilities in group work situations. The teachers used a general level when talking about assessment and the descriptions about what they assess were vague. In the teachers’ accounts of what is assessed it is possible to distinguish both the product and the process carried out at an individual as well as at a group level. Furthermore, the results reveal two different assessment strategies, that is how the assessment was carried out; by teachers (from outside the group) or by students (from inside the group). Additionally, the results disclosed that the teachers have difficulties in concretising and verbalising what and how they assess. The teachers also experienced uncertainty and contradictory demands concerning assessment of students in group work. They were concerned about this difficulty. Conclusion: The teachers were of the opinion that group work is used foremost to develop group work abilities and not as a means of acquiring academic knowledge. This influenced the teachers’ mode of using assessment, that is, they focussed on assessing the collaborative abilities. This article elucidates some of the teachers’ problems concerning assessment in group work and some pedagogical implications based on empirical findings. Research-based theories and models within these areas could increase teachers’ confidence, which, in turn, could increase good practice, their ability to use group work as a mode and also their competence in assessment.