Customer involvement in large scale, innovative and technologically challenging projects is typically intensive, reaching far beyond the classical customer roles of requirements definition and product verification. The involvement may include active participation of customer personnel in project design and in technical and program management decision making, blurring the line between the producer and the customer. Based on a concept of adapting management practices to project properties, the study proposes a contingency model regarding the effects of customer involvement on project success. A triple-stage research was conducted, combining findings from a qualitative multiple case study of aerospace projects in Israeli government-owned defense Industry; and a quantitative web-based survey of 155 projects. The study results augment the existing body of knowledge by a detailed analysis of customer representatives' involvement modes, focusing on the supervision-participation continuum, and the impact on project success, in various types of projects. The study proposes practical managerial guidelines for customers and producers organizations. The results of improving the way customers are involved in projects may be significant.