Briefly explain how long-run equilibrium is different for each of the following types of markets: perfect competition, monopolistic competition, and monopoly.

In both perfect competition and monopolistic competition, there are only small barriers to entry. Therefore, if existing firms are earning positive economic profits, new firms will enter the market, forcing profits of all firms down. In the long run, all firms in these markets will earn zero economic profits. In monopolies and oligopolies, on the other hand, there are strong barriers to entry that prevent some (or all) new firms from entering the markets. Therefore, firms in these markets will be able to earn long run profits because the barriers to entry will prevent the competition from entering the market and taking away some of their profits.