Firms often produce multiple related products. When products are substitutes, increasing production of one reduces demand for the other. When complementary, increasing one boosts demand for the other. Profit maximization requires jointly determining prices and output levels. A firm should produce quantities where the marginal revenue from each product equals marginal cost. It prioritizes producing profitable products up to the point where the least profitable product's marginal cost equals marginal revenue. Joint products have interconnected production and demand. Firms maximize profits by setting marginal revenue of each joint product equal to their shared marginal cost.