Teacher shortages are a phenomenon of growing international concern. The causes of this phenomenon are the focus of the research project What About Teacher Shortage (WATS up), which uses an international comparative perspective to analyse teacher shortages in Denmark, Germany, and Sweden. Because the nature of teacher shortages’ development is complex and influenced by several interacting factors, this theoretical article accounts for the complexity of the phenomenon and introduces a theoretical and methodological model for empirical research. The theoretical starting point is a policy problem approach; more precisely, one that adapts Bacchi’s model on focusing ‘what the problem is represented to be’ (WPR). Due to variations between the studied countries, a multiple case design is chosen as the methodological approach. This allows an individual analysis as well as a comparison of the countries. A methodological model comprising 8 parameters and 23 indicators summarizes the interacting factors influencing teacher shortages.