It is helpful at an exhibition to have a catalogue of the items on view. As this symposium is in effect a display of the operational aspects of marine radar, it may be of value in this paper to attempt to catalogue these aspects. They can be divided into two groups as follows:A. The aspects that determine the effectiveness of marine radar (the siting of the scanner and the operating policy followed, for example). These we propose to call the OPERATIONAL FACTORS.B. The benefits obtainable from marine radar (i.e. those aspects that may be said to measure its effectiveness—the time and worry saved, for example). For these we suggest the term OPERATIONAL YIELDS.