Since it was first described in 1763 Bayes' theorem has been applied, rejected and rediscovered in many fields. It's use in medical diagnostics is a relatively recent phenomenon. This talk will review the history of Bayes in medicine. We will then explore how Emergency doctors can practically apply these ideas in daily practice. How can we estimate pretest probability? How does Gestalt work? How can we use likelihood ratios to understand our diagnostic testing and the results? We will explore the threshold model of diagnostic reasoning and its application to patient-centred, shared decision making. Bayesian reasoning will be illustrated with common diagnostic dilemmas: subarachnoid haemorrhage, chest pain, cervical trauma, appendicitis, pulmonary emboli and tonsillitis.