The purpose of this thesis is to get an improved understanding of what the bases for relevance are, when people in real information need situations make relevance judgements. In order to meet this purpose, an investigation based on research literature is performed. Four types of relevance are investigated: topical or subject relevance, cognitive relevance or pertinence, situational relevance or utility, and socio-cognitive relevance. The investigation also deals with users relevance criteria identified in fourteen empirical studies, in which the users are adults and engaged in oral, textual, and multimedia information retrieval. Finally, the connections between the user criteria found and the types of relevance are discussed. The result shows that the user criteria that are brought together with topical relevance are few and fairly alike. The user criteria that are brought together with cognitive relevance and with situational relevance are much more numerous and of many different kinds. The criteria assigned to these two types of relevance are divided into groups. Among the criteria that are assigned to cognitive relevance, a part of the situational relevance criteria are to be found as well, depending on the catch-all nature of situational relevance. No connections between the user criteria and socio-cognitive relevance are found. The result indicates that relevance can be considered as a system and the relevance judgement as a process in which there are certain determined stages.