供给和需求是一个经济学模型，它被应用作决定市场均衡价格和均衡产量。这个模型适用于竞争性市场，而不适用于市场存在垄断或者寡头垄断的情况，需求或者供给价格分别跟消费者的需求量和生产者的供给量挂钩，形成市场两种力量决定价格和产量的均衡。模型的需求与供应都是经济学的基本概念：需求指大众因需要一件产品而产生的购买要求；而供应就指企业响应大众的购买需求而提供的产品供给。

它显示了随着价格升降而其它因素不变的情况下，某个体在每段时间内所愿意买的某货物的数量。

有一则古老的经济学笑话曾经讽刺道：只要教懂一只鹦鹉说“供给”和“需求”，牠也能成为经济学家。从中可见供给和需求是当代经济学一个核心课题，而这个课题亦常见于大多数经济学教科书。

In economics, the law of demand states that, all else being equal, as the price of a product increases (↑), quantity demanded falls (↓); likewise, as the price of a product decreases (↓), quantity demanded increases (↑). In simple terms, the law of demand describes an inverse relationship, and an elasticity, between price and quantity of demand. There is a negative relationship between the quantity demanded of a good and its price. The factors held constant in this relationship are the prices of other goods and the consumer's income. There are, however, some possible exceptions to the law of demand (see Giffen goods and Veblen goods).