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We examine in this paper the evolution of the concept of Mental Lexicon in Psycholinguistics. We oppose two main theoretical models of lexical access: search models and activation models. We show that activation models are actually clearly dominant within the current of connectionist approaches in psychology and neuroscience. In these approaches, lexical entries are considered as strongly related by facilitating and inhibitory links. These entries are not simply accessed but activated at different levels. The central notion of lexical entry has been strongly influenced by the consideration of the internal structure of the word at morphological level. Taking the morpheme as the basic unit of lexical entries leads to the proposal that lexical access procedures for morphologically complex words are combinatorial in nature. Combinatorial processing is clearly essential for sentence processing. Indeed, the meaning of a sentence is a function of the meaning of its words and the way they are syntactically combined. Thus, sentence production and comprehension require not only the activation of the sentence's words but also their integration within larger processing and representational units. To illustrate this point, we examine very briefly the MUC (Memory, Unification, and Control) model of language processing and its main neurobiological components.