The study of multienzyme systems has advanced considerably during the 1990s. The cell is more complicated than classical biochemistry presents it; it contains more structure, and the behaviour of any system of enzymes is more elaborate than can be explained in simple terms. Nevertheless, classical enzymology and metabolism remain central to any modification of the metabolic behaviour of organisms, as attempted by modern biotechnology and drug development techniques. In this work the authors draw attention to the drawbacks, problems, and opportunities associated with this field. The areas covered include problems with current practice, the imposition of human objectives on organisms, understanding health and disease, computer modelling, the increasingly complex picture of cell structure, control and regulation of metabolism, and the general contribution metabolic control is making to biochemistry on a broader canvas.