Evaluation of a firm always depends on its value items which characterize it. Values of firms always refer to the future, knowledge of the past and the present can, however, make the evaluation easier. Purchase and sale of a firm, or any form of cooperation, can be successful, it a firm has worked out a strategic plan, which enables it to prepare better for change in ownership. In the case of interest in a firm information is needed, which can be based on various analyses, on expectation of future returns, evaluation of risk etc. There can be various incentives for evaluation. Mergers - integration of two or more companies, which can be classified into horizontal, vertical and conglomerate ones - are one of them. As a suitable example of significance of mergers, in the table some important data on mergers in the U.S.A. in 1980s are given. An analysis of increase in economic profit or estimation of costs of distribution of the latter between two companies is an important condition of emergence of a merger. Goals of mergers can be miscellancous, e.g., scale economies in the case of horizontal and conglomerate mergers, economies from vertical integration, combination of complementary resources, utilization of surplus funds or also low efficiency of a firm, etc. Mergers always bring benefits to firms - otherwise the latter would probably not merge. The passed Law on Protection of Economic Competition in the Slovak Republic indicates several ways of dominance over a firm and the Antimonopoly office of the Slovak Republic is checking mergers in the spirit of this law.