This paper approaches the topic of Grammaticalization from a diachronic perspective, with particular emphasis on the development of some English modal auxiliaries. The first part contextualizes the topic within the filed of Cognitive Linguistics; the second, deals with a brief description of lexical and grammatical items. The third part of the paper provides some crucial information on the history of Grammaticalization. A final section outlines some of the mechanisms involved in the process of Grammaticalization together with the analysis of some English modal auxiliaries and the importance of it in when studying and understanding English as a foreign language.