Performance assessment of innovation projects is a central issue in innovation management research. Using existing literature, a model is developed to assess the performance of new product and new service development projects. In this model, project performance is defined as a combination of a formatively indicated operational performance construct and a reflectively indicated product performance construct. The validity of this model is tested based on a sample of 219 innovation projects assessed by innovation managers. Using only the innovation managers' responses, it is, however, not possible to distinguish between operational and product performance. The impact of common method bias and informant bias is subsequently assessed using a subsample of 128 of these 219 innovation projects that are assessed by the innovation manager and the project leader. These latter results show that operational and product performance are two distinct constructs. In addition, the multitrait–multimethod analyses show that especially the more abstract items of performance, such as the perceptions of quality, captured knowledge, competitive advantage, gained reputation, and customer satisfaction, suffer from random error and informant bias. Project leaders appear to be better informed to assess operational performance, while innovation managers are better in assessing product performance. The paper concludes with a qualitative comparison of several alternative performance models: the project performance model as derived from the literature, a similar (misspecified) reflective performance model, two stand-alone models in which operational and product performance are assessed separately, and a mixed model that uses a combination of innovation managers' and project managers' data. Based on this comparison, it is advised to use either the stand-alone models for operational performance and product performance or the mixed model whereby the project leader assesses operational performance and the innovation manager the product performance of an innovation project.