A pioneer and major thinker in the sociology of science, Joseph Ben-David wrote with refreshing directness on questions central to the history and sociology of science. As they are combined here, Ben-David's essays reveal the richness and synthetic power of his intellect in a way his separate publications never did. Two themes form the heart of Ben-David's ground-breaking work: the emergence, existence, and growth of science as a distinctive activity within society, governed by a specific "scientific ethos"; and the social construction both of new objects of scientific study and of new scientific disciplines. Ben-David argues that only where the scientist's social role is institutionalized (i.e., recognized as legitimate by society at large), can science as a sustained and continuous activity exist and thrive. By the same token, new scientific objects and disciplines emerge where social circumstances encourage and sustain new ("hybridized") social roles. Ben-David's is a distinctly historical sociology of science, providing a theoretical framework capable of integrating both the historical and the synchronic approaches; it is also complementary to the perspective of the sociology of scientific knowledge.