 The terminology evolved over the years. Carnap tended to think of his first engineering projects as “rational reconstruction”; later he favored the term “explication”. Both refer to the reconstruction or replacement of particular terms or concepts within our more primitive ordinary languages (or vestiges of them in scientific languages) rather than to the design and development of entire languages or language frameworks. There was a progressive change of emphasis over the course of Carnap’s career in the goals and the scope of this reconstructive engineering. In the early years Carnap (along with most of his colleagues in the 1920s Vienna Circle) saw these engineering projects in much the same light as the eighteenth-century Encyclopédistes and nineteenth-century positivists had seen them. Ordinary language and traditional concepts were to be overcome; they were to be replaced by better and more scientific ones. Later, Carnap’s view became more pluralistic and dialectical (Stein 1992); he increasingly recognized the practical nature of conceptual engineering and allowed more room for the standpoint of the language user, as well as the differing priorities and value systems involved in the choice of language frameworks or explications. Recently, Carnap’s idea of explication has attracted a good deal of attention (Maher 2007; Kitcher 2008; Carus 2007a; Dutilh Novaes &amp; Reck 2017; Brun 2016; several papers in Wagner 2012), particularly among authors seeking to define the proper role of the new subdiscipline of experimental philosophy (Justus 2012; Shepherd &amp; Justus 2015; Schupbach 2017) or of the methodology of mathematical philosophy (Leitgeb 2013). More details are to be found in the supplement on  Methodology.  (For more on conceptual engineering more generally, see, e.g., Cappelen 2018.)