As organizations continue to manage growth by developing a diverse portfolio of products and services in semiautonomous business units, they become increasingly fragmented internally. Such fragmentation results in highly diversified business processes for performing similar activities, leading to reduced operational efficiency, coordination, and information sharing. Horizontal business process integration entails change in temporal and spatial dimensions to mitigate this problem and identify common processes to help achieve synergies. Current research on Business Process Management has not paid much attention to enabling this activity. Motivated by this concern, the primary objective of my dissertation is: “how horizontal integration of business processes is achieved by semiautonomous business units to realize the benefits of better operational efficiency, information sharing, and coordination?” Using a two-phased approach I address this objective. In the first phase I develop a process theory of BPM in horizontal integration using grounded theory methodology. Also, this study identifies the rich contextual knowledge that is necessary to understand and reuse business process fragments. This study was conducted in a very large U.S. corporation as a part of an initiative to identify core processes in a multi-billion dollar supply chain process. Based on the findings of the first phase, in the second phase I develop a decision support system to aid process designers to help find similarities in process models. The effectiveness of the system for improving performance in business process modeling activities is evaluated using an experiment.