Significant progress of technologies related to safety of ships' structures and navigation safety systems, comprising techniques of communication in danger, ships' traffic control, reporting systems, positioning, navigational marking and rescue systems, has not been capable enough to let avoid many collisions or crashes which may happen at sea. Analyses of accidents on the Baltic Sea, such as the tragedies of "Jan Heweliusz" and "Estonia" ferries, proved that human life rescue actions have been characterized with a high risk and still low effectiveness. The main reason for relatively low effectiveness of search and rescue operations is lack of easily and quickly available, precise information about positions of ships in danger. Besides, onshore centres, responsible for coordination of rescue operations, are not acquainted with numbers of people on the ships. Thus, the automatic ship identification system, called AIS in brief, can be the element, which improves SAR effectiveness and reduces a number of probable collisions. The AIS system is a subject of this paper. The paper content is a description of AIS basic characteristics, the principles of operation and functions. Describing the system being implemented for Polish maritime areas, authors indicate possibilities of its application as an aid to navigation at sea.