One of the most powerful policy levers governments can use to adjust the supply of doctors and nurses to projected demand is the so-called numerus clausus, that is, the regulation of the number of students entering medical and nursing education programmes. This chapter describes the evolution of numerus clausus policies in OECD countries over the past 15 years and key challenges in achieving an adequate number and mix of different categories of health workers. Since 2000, most OECD countries have increased substantially the number of students admitted to medical and nursing education, in response to concerns about current or future staff shortages. This has often been accompanied by deliberate policies to increase more rapidly postgraduate training places in general medicine vis-á-vis other specialties to strengthen the primary care workforce. A number of OECD countries have also introduced or expanded training programmes for advanced practice nurses such as nurse practitioners also to increase access to primary care by relying more on non-physicians. Following a strong and steady expansion in training capacity, some countries now worry about a possible over-supply of graduates entering the labour market. How might governments use more wisely these numerus clausus policies and the large amount of public resources spent in training future health workers? This chapter stresses that the health workforce planning models that are guiding these policy decisions need to better factor changes in population health needs and in the scope of practice of different health care providers that might impact on their future demand. These models also need to take into account the growing international mobility of students and health workers, which makes it more complicated to determine the “right number” to train at a national level.