Book excerpt: Although entrepreneurship has become a buzzword in the public debate, a coherent definition of entrepreneurship has not yet emerged. In this paper, we review and compare the most common theoretical definitions of entrepreneurship in economics and discuss their connection to the various empirical measures in use. We argue that entrepreneurship is best considered a multifaceted concept, and that the different empirical measures reflect different aspects of entrepreneurship. The relevance of this exercise is illustrated by the fact that in a cross-country comparison of entrepreneurship, we find that the relative ranking of countries depends crucially on the indicator used.