In the literature on English tense usage, expressions of futurity such as(1) (a) I will/shall leave next week.(b) I'm going to leave next week.have already received a lot of attention, especially so in the pedagogical descriptive tradition of English linguistics (cf. Close, 1977; Haegeman, 1981, 1983; Leech, 1971; Quirk et al., 1985; Palmer, 1974, 1979; Wekker, 1976, etc.). Although these accounts are attractive, they raise further questions because most of them do not propose to deal with the problem against a formal theoretical background. As a consequence, the rules formulated to describe the use of shall/will or be going to in (1) tend to be intuitive and often do not really allow any decisive choice to be made in many instances of usage. On the other hand, Reichenbach type analyses of tense interpretation are usually mainly concerned with the general problem of tense representation and treat both examples in (1) as illustrations of future tense without detailed discussion of the contrasts between them.