  The use of lightweight formal methods (LFM) for the development of industrial
applications has become a major trend. Although the term "lightweight formal
methods" has been used for over ten years now, there seems to be no common
agreement on what "lightweight" actually means, and different communities apply
the term in all kinds of ways. In this paper, we explore the recent trends in
the use of LFM, and establish our opinion that cost-effectiveness is the
driving force to deploy LFM. Further, we propose a simple framework that should
help to classify different LFM approaches and to estimate which of them are
most cost-effective for a certain software engineering project. We demonstrate
our framework using some examples.
