This study addresses the relation of pre-school teachers' mathematics content knowledge and their performance--how they perceive mathematical learning situations and whether they are able to plan adequate actions that foster children's learning--in the informal settings of pre-schools. It thus addresses a serious gap in teacher research that has so far mostly been focussed on the formal settings of primary and secondary schools. The paper presents the instruments used to assess the knowledge (a paper-pencil test) and the performance (a video-based assessment), as well as the results of a study involving 354 prospective pre-school teachers. The results indicate that mathematical content knowledge is a significant predictor of the pre-school teachers' ability to perceive learning situations and to plan educational actions to foster learning. Such evidence not only supports the validity of the knowledge test, but it is also relevant for policy makers because it leads to conclusions about the important opportunities to learn that need to be provided during pre-school teacher training.