In this paper the problems of organization and representation of concept knowledge are addressed from an autonomous agent perspective. The first part of the paper discusses the question of why an autonomous agent needs concepts at all and what other uses it can make of them. After the necessary and/or desirable functions of concepts have been identified, previous attempts to represent concepts are briefly surveyed to see if they support these functions. The result of the survey is negative in the sense that none of these single and simple representations are powerful enough to meet the discussed requirements. Moreover, this result indicates that, to support all the necessary and/or desirable functions, a structured composite representation is needed. Such a structure is then suggested, followed by a short discussion concerning which kinds of representations are adequate for the different parts of the structure.