Sales derive from the behaviour and actions of consumers, customers, buyers and clients. Such behaviour is measured in terms of purchasing, repeat-buying, duplicate buying, retention and switching. An understanding of these measures and associated patterns and models is critically important. Practical uses of this knowledge are considered: auditing the performance of established brands, predicting and evaluating the performance of new brands, checking the nature of unfamiliar markets, of partitioned markets and of dynamic situations (for example, as a result of price-promotions, advertising, the introduction of loyalty-programs, and new product launches). Practical exercises illustrate the implications for understanding consumers, brands, the marketing mix and marketing management.