Various scholars have argued that knowledge processes in organizations are integrally linked in practice. The extant literature though treats them separately and thereby disregards the interactions and tensions between them. A result of this way of studying knowledge processes is that little is known about their relative importance and how they work together. This paper addresses this gap in the literature through a critical incident study of knowledge processes in product development projects of high-tech small firms. The paper starts off with a conceptual framework comprised of four knowledge processes—knowledge creation, knowledge application, knowledge integration, and knowledge retention—and their interactions. From the framework, three hypotheses are derived concerning the importance of these types of knowledge processes and their interactions, which in turn guide the empirical research. The hypotheses were tested in a retrospective study of 58 critical incidents in product development projects of 16 high-tech small firms in the Netherlands. Data were collected through semi-structured interviews using the critical incident interviewing technique. Interviewees were asked to “relive” and describe particular successful and unsuccessful examples of product development projects in the past. The analysis of the interview data focused upon whether there are differences between successful and unsuccessful projects in the types of knowledge processes and interactions that are performed. After coding all data into the various types of knowledge processes and interactions of the framework, t-tests were used to test for significance of differences. The findings indicate that the difference between success and failure in these projects lies primarily in the extent to which knowledge integration and integration between knowledge processes have taken place. These findings demonstrate that, of the four knowledge processes, knowledge integration had the most significant impact on product development project success. The study demonstrates furthermore that higher degrees of interactions between knowledge processes were also associated with project success. Despite the limitations of this study, these results provide empirical support for the claim that integration is a key factor in organizations in general and in innovation projects in particular. For academics, this suggests further research on knowledge integration, and integration between knowledge processes, is warranted. For practitioners, it means that integration is a key process to be considered when choosing and executing new product development projects.