Beyond the traditional focus on product innovation, prior research and practical examples from a variety of industrial settings underscore the importance of the early phases of process development and process innovation. Despite the potential for large cost savings and efficiency gains, however, little is known about what firms actually do in the early design and concept creation phases of process development, and what guides their subsequent formal process development efforts. By means of a longitudinal multiple case study of four large companies, we bridge this gap by conceptualizing a ‘process definition’. This process definition includes a process concept and is the ‘process equivalent’ of a product definition. Our analysis shows that firms create such process definitions through iterative trial-and-error processes, in which experiments, environmental scanning, and administrative planning constitute key methods for uncertainty reduction. Mainstream theory on product definitions fails to account for the key dimensions of a process definition. On the contrary, dimensions such as the understanding of production needs, assessment of product consequences, a thorough implementation plan and early anticipation of intended outcomes, constitute key dimensions of a successful process definition. These findings are particularly relevant to process development managers, plant managers, and others interested in process development and management of production processes.