What makes any investigative field a scientific discipline? This article argues that disciplines are ever-changing frameworks within which scientific activity is organised. Moreover, disciplinarity is not a yes or no proposition: scientific activities may achieve degrees of identity development. Degree of consensus is the key, and consensus on many questions (conceptual, methodological, institutional, and social) varies among sciences. Lastly, disciplinary development is non-teleological. Disciplines pass through no regular stages on their way from immature to mature status, designations articulated within the rhetoric of discipline formation. Scientists assemble disciplines using many elements: phenomena, methods, instruments, theories, analytical techniques, and institutional tools such as journals, government bureaus, and university positions. Scientists created geophysics during the nineteenth and twentieth centuries through such a combination. Whether geophysics became a discipline depends on how discipline is defined.