Increases in the stock of ideas possessed by societies are central to modern economic growth. The implications of idea flows are striking: Klenow and Rodriguez-Clare (2005) estimate world production would be just 6% of its current level if countries did not share ideas. Yet, although theoretical economists have studied ideas and their diffusion extensively, empirical studies are scarce because ideas are inherently difficult to measure. Previous empirical studies of idea flows have tended to use proxies such as trade flows, foreign direct investment, migration, and patent citations. However, with the exception of the latter, these measures are not pure idea flows, and do not capture the key properties of ideas, namely non-rivalry and disembodiedness. My research proposes a novel measure of idea flows, namely book translations, and uses it to study the factors that affect the international diffusion of ideas. Book translations are an attractive way to quantify idea flows because they are both non-rival and disembodied; they are a pure measure of idea flows rather than a by-product of a process such as trade or migration, and their key purpose is to make the ideas contained in the book accessible to speakers of another language. In chapter 2, I outline the economics literature on ideas and their diffusion. I motivate and discuss book translations as a measure of idea flows, and provide a framework for thinking about when translations are likely to occur. I describe the translation data in chapter 3. The source of the data is an international bibliography of translations collected by the United Nations Educational, Scientific, and Cultural Organization. From this bibliography, I compile a data set of over 2 million translations published in 80 countries since the 1949, including detailed information on each title translated. I then document the main patterns of translation flows. In chapter 4, I employ a gravity framework to study how distance affects translation flows between countries. This sheds light both on the barriers to international idea diffusion and on the underlying causes of the negative relationship between distance and trade. Translations differ from trade in that they have zero transportation costs, but they are subject to similar search and information costs and costs of forming contracts. I estimate a gravity model where bilateral translation flows vary with the sizes of the countries and the distance between them, and find the elasticity of translations with respect to distance to be between -0.3 and -0.5 for the 1990s; these values are significantly smaller than the equivalent elasticity for trade found in the literature, suggesting a significant role for transportation costs in the distance effect on trade. In addition, I present several pieces of evidence that suggest supply-side frictions play a larger role in the distance effect on translations than do consumer preferences. For instance, the speed with which titles are translated, which is likely to largely capture supply frictions as opposed to demand factors, decreases significantly with distance. Finally, in joint work with Ran Abramitzky (chapter 5), I study how the collapse of the Communist regime in Eastern Europe at the close of the 1980s affected the international diffusion of ideas. We show that while translations between Communist languages decreased by two thirds with the collapse, Western-to-Communist translations increased by a factor of seven and reached Western levels. Convergence was full in economically-beneficial fields such as sciences and only partial in culturally-beneficial fields such as history. The effects were larger for more Western-oriented countries. These findings help us understand how institutions shape the international diffusion of knowledge and demonstrate the importance of preferences in determining the type of ideas that diffuse into a country.