In this study the author discusses various theories that have been put forward to account for the choice between the present perfect and the preterite in expressions of past time in English. The distribution between the two verb forms is examined in a varied corpus consisting of more than 13,000 recorded verb forms, a little more than half of them from present-day English (British and American, spoken and written), the rest from earlier English all the way back to Old English. The analysis of the contemporary corpus is supplemented by elicitation tests carried out with British and American informants. It is argued that in the present-day language the alternation between the two verb forms is determined above all by the presence or absence of temporal adverbials and other contextual factors, considerations to do with current relevance playing only a subsidiary part. While many other writers have assumed that the rapid advance of the present perfect that took place in earlier English has continued up to the present day, investigation of the historical corpus shows that this advance has been arrested within the Modern English period, to the extent that the present perfect now seems to be losing ground to the preterite, especially in American English. An explanation is offered of why the development in English is so radically different from that observable in French, German and many other languages.