Among historians and sociologists of science, it is a commonplace that scientists' accusations of plagiarism defy straightforward explanations. To appreciate that these accusations need not indicate simply that given individuals have claimed others' ideas as their own, it is important to remember that scientists' habits of communication create specialized research communities in which professional practices are – and are intended to be – relatively standardized. A defining property of science is, after all, that it requires practitioners to reach consensus about solutions to their common research problems, in pursuit of which disciplinary colleagues are expected to exchange regular reports of their theories, information, materials and techniques. Certainly, patterns of behaviour do not fully realize the scientific ideal; there is no end of evidence that scientists’ communications may be less than fully frank, that their judgements are affected by diverse personal considerations, and that their research is shaped by peculiarities of places and things. But to find disparities between the real and the ideal does not mean that the ideal does not inform occupational practices. And because scientists belong to exchange networks, their standardized practices are virtually guaranteed to lead to so-called ‘simultaneous discoveries’. Sometimes, when two or more scientists reach a given conclusion at the same (or nearly the same) time, their agreement occasions mutual congratulations, since it confirms the appropriateness of scientists' shared interpretations. (When in 1858, for example, prominent British scientists celebrated Charles Darwin's and Alfred Russel Wallace's independent formulations of the concept of natural selection, they rendered the concept less controversial than it might have been had it been advanced by one man alone.) At other times, however, a scientific event that might be regarded as a simultaneous discovery under some circumstances instead becomes an occasion for expression of discord among interested scientific parties, prompting a ‘priority dispute’ or (worse) an accusation of plagiarism.