Teacher shortages have long been a policy concern for many countries worldwide. Despite wide-ranging policy initiatives and billions of pounds spent addressing the issue, shortages are still being reported, especially in the secondary sector and for some subjects. This recurrent teacher supply “crisis” is complex, and has no one simple cause or set of causes. The number of teachers in and required by schools is linked to a wide array of factors that include cohort birth rates, the supply of graduates overall and per subject area, the economic context (the relative attractiveness of teacher pay and conditions), curriculum demands, workload, teacher retention, changes to retirement and pension ages, and the subjective opportunity structure as it appears to young people considering careers. Most prior research has considered only a few of these factors in isolation, or as a snapshot of the overall problem. This tends to distort the relative importance of each factor and so gives misleading results. To address the complex issue, a multi-pronged approach is advocated, which looks into policy decisions, longitudinal data on teacher numbers and teacher vacancies, numbers applying to teacher training, individuals’ decision-making processes, and robust evaluations of policy interventions. This chapter describes how a combination of these methodological approaches is a better way to understanding, and perhaps helping find a solution to, the persistent shortages of teachers.