Although it retains a central position in the main theories of innovation, there are few studies that examine the factors that provide inducements for process innovation at the firm level. Using a large scale survey of UK manufacturing firms, we explore different explanations for why firms develop process innovations. Contrasting the sources of incremental and radical process innovation, our study indicates that there may be complementarities between them. We also find that firm size, the presence of formal research and development, and the use of suppliers as a source of knowledge all increase the chances that a firm will be a process innovator. This article discusses the implications of these results for future theoretical and empirical studies of the innovation process.