Doing the right innovation projects is critical to firm's success; therefore, academics and practitioners are striving for optimizing innovation project portfolio management (IPPM). Although some research on certain issues in IPPM has been conducted so far, valid empirical evidence on the use, outcomes, and most important success drivers of portfolio methods in innovation management is rare. We aim at discovering the cause-and-effects of IPPM performance. This paper shows that performance of IPPM may be better understood if it is considered as an integrated system of portfolio balance, strategic alignment, and value maximization simultaneously. Using in-depth data from 29 interviews in 12 companies, we use a grounded theory approach to develop a general model of what drives IPPM in detail and how these causes are related to effects on both project performance and firm performance. According to the findings from these qualitative data, effective IPPM is the result of three constructs: usage of IPPM methods, IPPM design, and project characteristics. These cause-and-effects are moderated by management perception and satisfaction with IPPM.Index Terms-Innovation project characteristics, innovation project portfolio, portfolio balance, portfolio methods, qualitative study.