Economists think at the margin — evaluating the incremental cost and benefit of one more unit — and this habit of thought transforms business decision-making. This article explains marginal analysis and shows why average thinking leads to poor choices. It covers applications from pricing strategy (marginal cost pricing) to hiring (marginal productivity of labor) to product development (marginal feature value). The article also warns about the 'marginal cost trap' that Clayton Christensen identified as the root of ethical failures.