Demand theory analyzes the relationship between demand for goods/services and their prices or consumer incomes. Demand is based on consumer needs, wants, and ability to pay. The quantity demanded of a product typically decreases as price increases, as shown by the downward sloping demand curve. Demand is influenced by factors like prices of substitutes and complements, consumer incomes and tastes, population levels, and price expectations. Exceptions to the inverse price-demand relationship include Veblen goods and speculative demand for assets. Demand theory is foundational to microeconomics.