Market Structure refers to characteristics determining firm competitive behavior: number of firms, product differentiation, entry barriers, pricing power, profit levels. Four main types: Perfect Competition (many firms, homogeneous product, free entry/exit, price-taker). Monopoly (one firm, unique product, high entry barriers, price-maker). Monopolistic Competition (many firms, differentiated products, free entry/exit, some pricing power). Oligopoly (few large firms, differentiated/homogeneous products, entry barriers, interdependent pricing). Characteristics determining structure: Number and size of firms, Product differentiation, Entry and exit barriers, Information availability, Pricing behavior. Impact on outcomes: Perfect competition yields efficient allocation; monopoly yields allocative inefficiency (higher prices, lower output, deadweight loss). Intermediate structures between extremes. Regulation role: Government may regulate monopolies, encourage competition, address market failures. Indian examples: FMCG is monopolistic competition; telecom is oligopoly; railways were monopoly, now regulated. ICAI tests: Identifying structures, understanding characteristics, comparing outcomes. Exam tip: Remember "perfect competition = efficient," "monopoly = inefficient"; use this to evaluate statements.