Being described as the Cinderella of second and foreign language acquisition research, the acquisition of lexical knowledge depth is still in its embryonic stages. The existence of no complete theory of vocabulary acquisition and no comprehensive approach for vocabulary teaching and testing could be the best proof of the aforeentioned claim. Furthermore, the partial understanding of the different cognitive and linguistic factors in this process led the present research to aspire to contribute in meeting the need suffered in this area, focus being on the Moroccan educational context. The present book emraces a PhD thesis that attempted to shed light on the nature of the development of three EFL lexical knowledge aspects-meaning, synonymy and collocation- for Moroccan learners. The different statistical measures run using the SPSS software suggest that there is indeed a "natural", predctable order of development, that L1 plays a pivotal role in this process and that the linguistic tasks, receptive or productive, affect the L1-EFL mental lexical connectivity respectively. All these factors hampers the desired native-like use of EFL lexical knowledge.