Modern practical methods applied in developed market economies are almost immediately implemented in the Polish financial institutions. However, in the Polish professional writing in banking and finance, the absence of theoretical discussion on relations between financial institutions and non-financial institutions can be observed. It concerns in particular theory of financial contracting, agency theory, and associated with them, theory of corporate control (corporate governance). Insufficient knowledge of those theories makes difficult appropriate understanding of mechanisms of modern capital markets, including mergers and acquisitions as well as understanding of the relationships between banks and companies.

The main aim of the paper is to present introductory theoretical foundations of analysis of financial intermediary institutions (financial intermediaries). Market imperfections - transaction costs, information asymmetry and state regulation are exposed as the causes for formation of financial intermediaries. Financial intermediaries can reduce transaction costs in the following ways: as market makers, due to economies of scale as a consequence of specialisation in information gathering and processing, due to economies of scope stemming from diversification of their activities, by reducing costs associated with matching potential lenders and borrowers, due to expertise in acquiring appropriate legal advice necessary in preparing financial contracts and due to modern computer technology allowing to provide sophisticated service at reduced costs. The consequences of information asymmetry - moral hazard, adverse selection and credit rationing are also discussed.

The paper includes description of rudimentary problems and an introduction to further research, and in particular to the studies of mathematical models of presented concepts, which cannot be adequately analysed otherwise. Due to numerous difficulties in translating Anglo-Saxon banking and finance terminology into Polish, presentation of some terminological proposals has become an additional purpose of the paper.