1. Barriers to entry protect monopolies from competition and allow them to maintain supernormal profits in the long run. Barriers include economies of scale, natural monopoly conditions, product differentiation, ownership of distribution channels, and legal protections. 2. Monopolies may engage in limit pricing by charging below the short-run profit-maximizing price to deter potential new entrants from entering the market. However, very large profits still attract new entrants. 3. While monopolies are inefficient compared to perfect competition, they can drive innovation through research and development spending and face constraints from corporate takeovers if they become too inefficient.