This research investigates three issues related to the economic performance of oligopolistic markets where firms produce differentiated products and compete in prices. First of all, this dissertation uses a Markov Perfect Equilibrium approach with fixed periods of commitment of actions to answer the question of what prices a duopolists will charge in equilibrium if they produce horizontally differentiated products, move alternatingly, and compete in prices forever. It is found that firms charge prices which are higher than Nash equilibrium prices but lower than the fully collusive equilibrium prices. Also, contrasted with the Nash equilibrium of the one-shot constituent game, the firm having the significantly higher demand responsiveness to its own price always charges a lower price than the other firm does although it has higher marginal cost. The dissertation then proceeds to study whether a firm can overcome its cost disadvantage by upgrading its product over the rival's, and if so, whether there exists a profit-division which will induce the low cost firm and the high cost firm to collude and no one has an incentive to cheat. The results show that (1) the ability of upgrading the product over the rival's can allow a high cost firm to earn higher profit than a cost advantaged low cost firm; (2) there exists at least one profit-division which can sustain full collusion; and (3) in the collusive equilibrium firms enlarge their quality differences to alleviate the price tension between their products. Finally, this work investigates the welfare effect of mergers which occur in an oligopolistic industry where firms produce differentiated products. It is shown that for the merger to be socially beneficial, the number of the merging firms must be less than the total number of firms in the industry minus the ratio of the products' own elasticity to cross elasticity. The analysis indicates that the welfare effect of a merger of a specific size depends on the substitutability among products of the industry.