The development of international and national commercial relationships, determine the need to use more and more frequently the services of intermediaries. According to the Civil code of Republic of Moldova, one of the area which is covered by commercial intermediation is the activity of intermediation in securities sailing. Within the framework of the present article, not only the activity particularities of professional participants on the securities market (brokers, dealers, and underwriters), the legal nature of their relationships with their clients and third parties, but also the legal requirements of their authorization and activity, is undertaken. For all this, there are made references to the relevant doctrinaire opinions and national legislative acts.