It is a truism that all languages are subject to contact with other linguistic systems and, therefore, that they may undergo external influence. English, in particular, has been subject to various contacts since the earliest Germanic invaders settled in Britain. The role of contact and its extent varies from one case to another, depending especially on social conditions and on the linguistic systems involved (including their typological distance); regarding the social aspect, relevant factors are the length and the intensity of contact, the actual amount of bilingual speakers, their proficiency, their attitude to the different linguistic systems, etc. If it is possible to analyse contemporary contact situations (including major instances such as pidgins and creoles) from these points of view, much more difficult or even impossible is it in the case of historical situations which occurred in the past. One such instance is represented by the Old English varieties, which experienced, first, contact with indigenous Brittonic types of speech, then with Scandinavian ones, and, finally, with Norman French after 1066. After Keller’s study (1925), the ‘Celtic hypothesis’ has been revived in the last few years (see, e.g., Tristram 1999, 2000, 2002, 2003, 2004, Filppula et al. 2002, Lutz 2002, etc.). In particular, Tristram (2002 and 2004) argues that the indigenous Celtic varieties spoken in Britain influenced the imported Germanic ones so deeply as to cause the typological shift of English from synthetic to analytic; on the other hand, White (2002 and 2003) claims that contact with the Britons made inflections more and more marginal and, consequently, since they had become “ornamental” and communicatively useless items, they were dropped; this, in turn, brought about a sequence of phonological changes which blurred phonological oppositions in unstressed syllables and eventually resulted in the merger of unstressed vowel under 'schwa'. These are very stimulating suggestions, and, as Tristram rightly claims, we should not underrate this type of foreign influence on Old English. In this paper, we briefly consider the sociohistorical situation and, in more detail, the linguistic evidence in an attempt to demonstrate that the typological shift that English underwent is basically the consequence of internal factors, to which foreign influence may have contributed only marginally and, above all, mainly as an ‘accelerating’ factor.