This chapter examines some psycholinguistic issues raised in the study of language understanding. Questions concerning the structure of the mental lexicon and the nature of the computations that allow access to information stored in this lexicon are addressed. Research using the metaphor of lexical space and exploiting the lexical databases for Dutch and English is presented. The analysis of the similarity neighborhoods of words of different frequencies revealed a surprising pattern; common words have more neighbors than rare words and these neighbors are more frequent than those of rare words. This finding, when confronted with the word frequency effect, creates an apparent paradox that this paper attempts to resolve.