The major change in medical decision making over the last 50 years has been the realisation that treatment decisions would be improved if doctors’ existing knowledge was supplemented by evidence generated systematically through health services research. This paper discusses this changing paradigm and explains the related activities of evidence-based medicine, comparative effectiveness research, and health technology assessment. The latter is particularly important for making decisions on the provision of healthcare at the population level. The key steps in undertaking health technology assessments are explained, focussing on the types of literature they generate.