We measure the quantitative importance of labor mobility as a vehicle for the transmission of knowledge and skills across firms. For this purpose we create a unique data set that matches all applications of Danish firms at the European Patent Office to linked employer-employee register data for the years 1999-2002. The Danish workforce is split into "R&D workers", who hold a bachelor's or a master's degree in a technical field, and "non{R&D workers". We find that mobile R&D workers ("R&D joiners"') contribute more to patenting activity than immobile R&D workers. Furthermore, R&D workers who have previously been employed by a patenting firm ("patent exposed workers") have a larger effect on patenting activity than R&D workers without this experience. Patent exposed R&D joiners constitute the most productive group of workers: for firms that patented prior to 1999, one additional worker of this type relates to an increase in the number of patent applications of the new employer by 0.0646. This corresponds to a 14 percent increase in the mean number of yearly patent applications. We also find that mobility of R&D workers increases the joint patenting activity of the donor and recipient firms, confirming the importance of labor mobility for innovation in the economy.