In this article I suggest a tripartite classification of scientific activity; discovery, pursuit, and justification. I believe that such a classification can give us a more adequate description of scientific practice, help illuminate the various roles that evidence plays in science, and may also help to partially resolve differences between “constructivist” and “epistemologist” views of science. I argue that although factors suggested by the constructivists such as career goals, professional interests, utility for future practice, and agreement with existing commitments do enter into pursuit, it is experimental evidence that is decisive in justification. I illustrate this with two case studies from the history of contemporary science, experiments on atomic parity violation and their relation to the Weinberg-Salam unified theory of electroweak interactions and the fifth force in gravity. I also answer some of the criticisms offered of my earlier account of the episode of atomic parity violation.