This is a chapter from the free textbook "Economic Principles in Cell Biology". Cellular metabolic systems can be viewed as economic agents that rationally allocate limited intracellular resources, such as nutrients and enzymes, in order to achieve growth and other cellular functions. This chapter reviews and systematizes recent theoretical work that applies microeconomic concepts, such as budgets, prices, shadow prices, and substitution versus complementarity, to resource allocation in cells, and presents them as a unified “microeconomics of metabolism”. By casting intracellular resource allocation in economic terms, the general framework offers intuitive interpretations and quantitative predictions for cellular metabolism and growth: in particular, how metabolic reac- tions and growth change in response to changes in nutrient availability or to pharmacological interventions involving metabolic inhibitors. Remarkably, we use a microeconomic view and the physicochemical constraint known as the law of mass conservation to abstract from molecular-biological specifics to a universal, phenomenological understanding of cell metabolism: because multiple distinct precursors must be available together to produce biomass and other essential metabolites, having an excess of one compound is useless without the others. We demonstrate that this property generally yields a linear response relation under small perturbations, and gives rise to diminishing returns under larger changes in nutrient supply.