George Davey Smith is a clinical epidemiologist who has focused on methods for improving causal inference in studies of disease aetiology and disease prevention. His work has involved early implementation of ‘negative controls’ in epidemiological studies, the use of cross-context comparisons, sensitivity analyses, unobtrusive data collection methods and randomized trials in thought-to-be difficult situations. He pioneered the use of germline genetic variants for investigating modifiable causes of disease (‘Mendelian randomization’), developed several extensions of the basic method, and contributed to its application in many settings. He is an advocate of the pre-specified application of a range of methods, with different structures of potential biases, to the same question (‘triangulation’), as the key approach to strengthening causal inference. Throughout his career he has promoted increasing the accessibility of data, and implemented this in studies he has led, including the Avon Longitudinal Study of Parents and their Children (ALSPAC).