How do licensing and technology transfer influence the spread of General Purpose Technologies? To answer this question, we analyze the diffusion of the transistor, one of the most important technologies of our time. We show that the transistor diffusion and cross-technology spillovers increased dramatically after AT&T began licensing its transistor patents along with symposia to educate follow-on inventors in 1952. Both these symposia and the licensing of the patents itself played important roles in the diffusion. A subsequent reduction in royalties did not lead to further increases, suggesting that licensing and technology transfer were more important than specific royalty rates.