Design-based research (DBR) develops solutions in combination with research and has gained increasing popularity in educational research. This research approach involves close collaboration between researchers and practitioners. Many studies show that teachers are often passive participants in DBR projects, where their role becomes implementing researchers' ideas. This paper reports on a case where teachers became, instead, a driving force in a DBR project and investigates the conditions for this to happen. Data were collected by means of group interviews with the eight pilot teachers in the KreTek project, which was undertaken as a collaboration between a university and a municipality in Norway. The authors' own experiences as participants in the project also form part of the empirical basis for the study. The conditions for active involvement identified here are 1. A shared and relevant challenge; 2. An open entrance to the project; 3. An allocated time and meeting space; and 4. Responsibility and autonomy. The results are discussed in light of perspectives on teacher professionalism, theories on self-determination and previous research on teachers' roles in collaboration between researchers and teachers. It is concluded that by fulfilling the identified conditions for active involvement of teachers with an emphasis on trust, responsibility and autonomy, teachers may play an active role in DBR projects. In turn, this can facilitate a fruitful process where products of research projects using a DBR approach are deeply grounded in practitioners' contexts and thus are able to meet the actual needs of schools.