In recent decades many sociologists and philosophers of science, especially the so-called ‘new experimentalists’, have stressed the need for detailed studies of real, ongoing experimental practices, and have claimed that a new image of science results from such an approach. Among the new objects of interest that have emerged from laboratory studies, an important one is the tacit dimension of scientific practices. Harry Collins, in particular, has insisted that irreducibly tacit presuppositions and skills are inevitably involved in experimental practices, and (...) that these tacit resources play an essential role in the stabilization of successful scientific achievements. What I will call the ‘opacity of experimental practices’ has been analyzed in different ways, but on the whole, it has been claimed to have harmful epistemological consequences with respect to crucial issues such as the nature of experimental facts, scientific realism, scientific rationality, and the contingency of what acquires the status of an established scientific result in practitioners’ eyes. Such claims remain highly controversial. The aim of this paper is to discuss the epistemological implications of the involvement of tacit resources in experimental practices and to provide helpful conceptual tools with respect to the issue of tacit resources in science. Specifically, the paper provides an analytical clarification of the relation between the opacity of experimental practices and the principle of experimenters substitutability, the latter being a principle commonly viewed as a necessary feature of any good science. Serious doubts are finally raised about the validity of the experimenters substitutability principle, and the impact of these doubts with respect to the contingency of scientific facts and results is considered. (shrink)