This paper surveys what is known about the extent of international technology diffusion and channels through which technology spreads. Productivity differences explain much of the variation in incomes across countries, and technology plays a key role in determining productivity. The pattern of worldwide technical change is determined largely by international technology diffusion because a few rich countries account for most of the world's creation of new technology. Cross-country income convergence turns on whether technology diffusion is global or local. There is no indication that international diffusion is inevitable or automatic, but rather, domestic technology investments are necessary. Better understanding of what determines the effectiveness of technology diffusion sheds light on the pace at which the world's technology frontier may expand.