A merger occurs when one firm assumes all the assets and all the liabilities of another. The acquiring firm retains its identity, while the acquired firm ceases to exist. A majority vote of shareholders is generally required to approve a merger.

A merger is just one type of acquisition. One company can acquire another in several other ways, including purchasing some or all of the company's assets or buying up its outstanding shares of stock.

In general, mergers and other types of acquisitions are performed in the hopes of realizing an economic gain. For such a transaction to be justified, the two firms involved must be worth more together than they were apart. Some of the potential advantages of mergers and acquisitions include achieving economies of scale, combining complementary resources, garnering tax advantages, and eliminating inefficiencies. Other reasons for considering growth through acquisitions include obtaining proprietary rights to products or services, increasing market power by purchasing competitors, shoring up weaknesses in key business areas, penetrating new geographic regions, or providing managers with new opportunities for career growth and advancement. Since mergers and acquisitions are so complex, however, it can be very difficult to evaluate the transaction, define the associated costs and benefits, and handle the resulting tax and legal issues.

When a small business owner chooses to merge with or sell out to another company, it is sometimes called "harvesting" the small business. In this situation, the transaction is intended to release the value locked up in the small business for the benefit of its owners and investors. The impetus for a small business owner to pursue a sale or merger may involve estate planning, a need to diversify his or her investments, an inability to finance growth independently, or a simple need for change. In addition, some small businesses find that the best way to grow and compete against larger firms is to merge with or acquire other small businesses. Also, Reverse Takeover is one of the methods of Merger and Acquisition for a small business.

In principle, the decision to merge with or acquire another firm is a capital budgeting decision much like any other. But mergers differ from ordinary investment decisions in at least five ways. First, the value of a merger may depend on such things as strategic fits that are difficult to measure. Second, the accounting, tax, and legal aspects of a merger can be complex. Third, mergers often involve issues of corporate control and are a means of replacing existing management. Fourth, mergers obviously affect the value of the firm, but they also affect the relative value of the stocks and bonds. Finally, mergers are often "unfriendly."