Prescriptivists have long proscribed sentence-initial And (SIA), and sentence-initial But (SIB). However, SIA and SIB are increasingly used in newspapers and style guides have softened strictures against their use. Moreover, SIA and SIB are amongst the most frequently occurring sentence-initial connectives within their respective semantic groups of additives and contrastives. Given their use despite prohibitions, this paper examines the patterns of occurrence and function of SIA and SIB in academic writing. The data come from 1 million words of academic prose: 11 journals representing science, social science, and humanities. The data confirm the findings of Biber et al. (1999) that while coordinator and is more frequent in academic prose than but, SIA is much less frequent than SIB. The data also reveal a marked difference between low SIA and SIB occurrences in scientific writing and much higher occurrences in social science and humanities. Plus, SIA is the preferred additive connective compared with moreover, furthermore, and in addition, etc., and SIB is the second most preferred contrastive connective after however. SIA and SIB in academic writing function in three very similar ways: (i) to mark off a discourse unit by indicating the last item on a list, (ii) to indicate the development of an argument, and (iii) to indicate a discontinuity or shift with a previous discourse unit. Whereas the most common function of SIA is that of indicating the last item on a list, the most common use of SIB is in the development of arguments. SIA and SIB perform special functions that the alternatives of asyndetic or “zero” coordination, the use of similar discourse markers: moreover, furthermore, in addition, and however, respectively, or intrasentential coordination cannot perform. These special functions are derived from their particular semantic meanings, their role as coordinating conjunctions, and their reduced phonological prominence. These features allow SIA and SIB to preface a wider range of lexico-grammatical units such as interrogatives, stance adverbs and other discourse connectives and to create a tighter form of cohesion. It is these special features of cohesion rather than a move to colloquiality which are held to explain the occurrence of SIA and SIB in academic writing.