This document summarizes key concepts about firms operating in a perfectly competitive market. It defines a perfectly competitive market as having many small buyers and sellers of identical products, with no barriers to entry or exit. Firms are price takers while markets determine prices. The market equilibrium price is set where demand and supply intersect. Firms seek to minimize costs by producing where marginal cost equals average cost, and maximize profits by producing where marginal revenue equals marginal cost. Firms can earn profits, normal profits or losses depending on whether their average costs are below, equal to, or above prices and revenues.