This article presents a semantic-pragmatic analysis and comparison of the be V-ing construction and the emergent progressive aspect markers (PAM) be in the middle, midst, process or act of V-ing. On the basis of data from the Corpus of Contemporary American English, I first argue that the PAMs be in the middle, midst, process or act of V-ing show striking similarities with semi-auxiliary be V-ing. Both progressive constructions pattern with the same range of predicate classes, namely predominantly atelic actions and telic accomplishments. Not only is there overlap in their lexicalcollocational preferences, the emergent PAMs and be V-ing also impose the same grammatical reading on the sentence: they represent or "actualize" the situations expressed by the following V-ing as unbounded and durative, regardless of whether these aspectual properties are already inherent in the predicates or not. Second, I address the question in which respect the emergence of the be in NP of V-ing aspectualizer construction causes reorganization within the paradigm of progressive aspect. The difference between be V-ing and the PAMs lies in the expanded uses of the former and the specialization of the latter into expressions of continuousness, with the exception of be in the process of V-ing. This particular construction seems to have developed into an aspectual marker of imminence, meaning that it refers to the preparatory phase leading up to the action. In most instances of be in the middle, midst and act of V-ing, however, the PAMs indicate that a situation is taking place without interruption at the temporal zero point. Be V-ing does not really display this preference for continuousness, as it takes on as often other shades of progressivity, for instance implied iterativity or habituality, and non-aspectual functions such as prospectivity and subjective meanings. I claim that it is exactly this functional indeterminacy of the be V-ing form that probably triggered the development of the PAM as a construction reinforcing one specific feature of progressivity, namely pure continuousness.