Professor Sir Leslie Fowden was an organic chemist and one of the most influential plant scientists of his generation. He pioneered the new field of phytochemistry and was the world authority on plant amino acids. He was distinguished for the identification, characterization and biological function of a new category of nitrogen containing molecules in plants, the non-protein amino acids. During his research career, he isolated at least 50 of these compounds from a wide range of different plant genera and established their structural diversity, metabolic and toxic functions, and their scientific significance. In achieving this, he developed novel techniques for the rapid, quantitative separation of compounds by chromatography and electrophoresis, which were widely adopted in other disciplines. As Director of Rothamsted Experimental Station, he opened up new areas of research in plant biochemistry and molecular biology, and became an effective voice for the importance of agricultural research in the UK. He provided strategic leadership in the translational application of plant biochemistry to the agrichemical industry and to global crop resistance and production. His discoveries established the complexity and central role of plant nitrogen metabolism in plant growth and laid the foundations for current research in plant sciences aimed at improving both food security and the nutritional value of plants.