Most junior doctors learn common practical procedures, like lumbar puncture, on the job. This usually involves unstructured observation, demonstration and then supervised practice. Thus, most doctors have learned to perform a lumbar puncture without studying the theory of the procedure in any detail. The result is that patients experience less than optimal care, and more complications than necessary. This article first outlines some educational principles with regard to teaching and learning practical procedures and then describes the theory of diagnostic lumbar puncture.