Diffusion refers to the spread of ideas and behaviors across a population. The field does not cohere around a central question whose solution would clarify fundamental issues. Instead, what we see is the constant and fruitful exploration of new social worlds, and the adaptation of research methods and theoretical concepts to capture central realities of these worlds. We begin by describing two lines of diffusion research whose core assumptions and analytic strategy are diametrically opposed: network models of online behavior and translation studies of organizational fashion. We then survey contemporary trends in organizational research that broaden the scope of diffusion studies, focusing, in particular, on the study of politically contested practices that pit the interests of organizations and elites against various stakeholders such as current and prior employees, customers, and the public at large.