In the competitive global economy, increasing customer value by improving software product quality is the aim for organizations. Academia and industry are collaboratively striving to find new ways to achieve this goal. The last two decades have seen a mind shift where emphasis is now on improving processes to improve the quality of products. This approach has its roots in the realization that a better development process would lead to a better quality product. Based on this process improvement frameworks were created. These frameworks enable organizations to systematically improve their development processes and strive towards the ultimate goal of improving product quality. However, these initiatives were not always successful. The deriving factors that make the adoption and implementation of such frameworks a success were still unknown. In this regard, several primary studies have been conducted to identify the factors that make software process improvement initiative successful. Researchers have been investigating factors that affect success of Software Process Improvement (SPI). There was still a lack of a secondary study where systematic approach has been used to find factors that affect the success of SPI. In our thesis project we present results of the systematic review to accumulate success factors for SPI. These findings were validated by conducting an industrial survey. With our secondary study we have identified 31 important success factor categories of SPI. We have contributed to the body of knowledge by devising formal definitions for important factors. We have also unveiled the relationship of SPI success factors not only to different SPI framework but also to software process areas.