Enzymes are the astonishing, tiny molecular machines that make life possible. Each one of these small proteins speeds up a single chemical reaction inside a living organism many millionfold. Working together, teams of enzymes carry out all the processes that collectively we recognize as life, from making DNA to digesting food. This Very Short Introduction explains the why and the how of speeding up these reactions--catalysis--before going on to reveal how we have evolved these catalysts of such extraordinary power and exquisite selectivity. Paul Engel shows how X-ray crystallography has revealed the complex molecular shapes that allow enzymes to function at an extraordinarily sophisticated level. He also examines medical aspects of enzymes, both in the way faulty enzymes cause disease and in the way enzymes can be used for diagnosis and therapy. Finally, he looks at the many varied ways in which individual enzymes, taken out of their biological context, are used nowadays as tools--in washing powders, food production, waste treatment, and chemical synthesis.