John D. Lawson died at the beginning of this year. He is well known for his derivation of an equation that defines the conditions needed for a fusion reactor to reach a positive energy balance. The criterion was published in January 1957 and is universally known as the “Lawson criterion”. Fusion was not Lawson’s main research activity, but he made key contributions. While he spent most of his working life working on particle accelerators, he was involved in fusion research in the 50’s when he produced his famous criterion and then again from 1975-1976 when he worked at Culham on a design study of aconceptual fusion power reactor based on the reversed field pinch principle.