The topic of the English modal verbs is a very complex and wide-ranging field. They show such unique characteristic features that draw a well-defined borderline between the modals (and primary verbs in auxiliary function) and the class of full or lexical verbs. In the first chapter I will describe the behaviour of the modals on purely syntactic grounds and I will also try to make a distinction between the three main categories of the verbs. These features form a basis for the discussion of their present-day behaviour and for a comparison of their status in Old and Middle English.