In this chapter we will study the behavior of consumers. A consumer is an agent making decisions on consumption. It may be an individual, a household, a community of apartment owners in a building, etc. as long as they share a common objective. The problem of deciding what to consume and how much arises because resources are scarce. This means that the consumer faces restrictions when taking the consumption decision. We summarize these restrictions into a single one described by the income available to the consumer.