The Copley Medal is awarded to Dr R. Hill, F. R. S., in recognition of his pioneering contributions to the understanding of the nature and mechanism of the main pathway of electron transport in photosynthesis. Almost fifty years ago Hill made the first important discovery that allowed detailed chemical analysis of the pathways of photosynthesis, when he demonstrated the light-driven oxidation of water by isolated chloroplasts, and this made it possible to study water oxidation separately from carbon-dioxide reduction. This was the starting point in the elucidation of the electron-transfer pathway in photosynthesis, and in 1951 Hill, with R. Scarisbrick, uncovered the first component in the chain when they discovered cytochrome and established its key properties. Subsequently, with H. E. Davenport, Hill discovered the second component of the chain, shown later by others to be ferredoxin. With F. Bendall he formulated the ‘Z-scheme’ to describe the mechanism of electron transfer in photosynthesis in chloroplasts, which showed the relation between the photochemically driven elements and conventional electron-transfer chains found in other biological systems. This proposal brought great clarity to the field and set the scene for further detailed elucidation of the mechanisms.