In this historical essay an attempt is made to discuss the problem of decisive experiments both from the point of view of History of Science and of Philosophy of Science. The first part deals with Francis Bacon's idea of instantiae crucis and with the use of the term experimentum crucis mainly in optics. With respect to the experimental confirmation of Maxwell's electrodynamics the Duhem‐Quine Thesis is discussed. Duhem had argued that not a single hypothesis but only a complete theory is (...) examined by experiment. So a single experiment neither can prove nor can disprove a single hypothesis. With regard to Bucherer's and Neumann's data concerning the velocity‐dependence of the electron's mass the question of the certainty of conclusions arising from experimental tests is treated. In the last parts the really historical problem of the decisive experiments is considered, namely the gap between the context of design of an experiment and the context of evaluation of an experiment in retrospect. The examples here are the Michelson‐Morley experiment, the Franck‐Hertz experiment, and the Compton‐Effect. In the conclusion parity violation is discussed. Perhaps due to the possibility of a single alternative in theory and an unambiguous result of the experiment this test really was crucial. In general, however, the experimentum crucis will prove to be a very seldom event. (shrink)