Models of endogenous growth have strong empirical predictions about the determinants of technological progress. This thesis details the implications of alternative R&D-based endogenous growth models, and then surveys the empirical literature that tests different aspects of this New Growth Theory. Numerous studies attempt to test the validity of endogenous growth models but come to very different conclusions, since varying hypotheses are considered. There are few rigorous and plausible empirical assessments of whether the determinants of technological progress conform to the predictions of the theory. I provide new evidence on the relevance of R&D intensity for economic growth, using dynamic panel data methods, thereby contributing to the empirical literature that finds support for R&D-based endogenous growth models.