This study investigated biology teachers' (N = 148) understanding of models and modelling (MoMo), their model-related teaching activities and relations between the two. A framework which distinguishes five aspects of MoMo in science ("nature of models," "multiple models," "purpose of models," "testing models" and "changing models") served as a theoretical background. Teachers' understanding of MoMo was assessed using constructed-response items which were analysed qualitatively based on a coding scheme. The biology teachers mainly expressed a limited understanding of models as copies or idealised depictions used to show or to explain something. Model-related teaching activities were assessed through rating-scale items. The findings propose that models are primarily generated in biology lessons to show or to explain something but are rarely contrasted with other models, evaluated and modified. Significant correlations between teachers' understanding of the aspect "testing models" and their intensity of model-related teaching activities were found. This suggests that the aspect "testing models" is a key aspect of promoting teaching activities related to MoMo in biology lessons. The findings are discussed with respect to relevant literature about MoMo in science education and educational implications are provided.