This working paper examines the origins and development of the Koç Group, which grew to be the largest business group in Turkey. This enterprise was an important actor in the emergence of modern business enterprise in the new state of the Republic of Turkey from the 1920s. After World War II it diversified rapidly, forming part of a cluster of business groups which dominated the Turkish economy alongside state-owned firms. This study shows how the founder of the Group, Vehbi Koç, formulated his business model, and analyzes how his firm evolved into a diversified business group. The research supports prevailing explanations of business groups which identify the role of institutional voids, government policies and contact capabilities, but it also builds on and extends earlier suggestions in both the management and business history literatures that entrepreneurship needs incorporating more strongly as an explanatory factor. This working paper argues that Koç acted as both a Kirznerian and Schumpeterian entrepreneur to build his business group, both in its formative stages and later in its subsequent growth into a diversified group.