A dictionary is a necessary component for natural language processing. Yet there are different kinds of dictionaries (paper, electronic, mental) and in terms of efficiency they are not at all equivalent. Overall the best dictionary is the one that we carry with us everyday, the mental lexicon, and it is only when we lack a term or have word access problems that we reach for their paper or digital counterparts. Given the superiority of the mental lexicon we consider building electronic dictionaries according to similar principles, which supposes that we know what these principles are. To enumerate some of them is the goal of this paper. Unfortunately, the problem is too complex to be addressed in its full scope. Therefore we describe only a subset of the relevant work in psycholinguistics.