This paper analyzes the causality between inventor productivity and inventor mobility. The results show that the level of education has no influence on inventor productivity. Making use of external sources of knowledge, on the contrary, has a significant effect on productivity.Finally, firm size has a positive impact on productivity. Firm size also influences inventor mobility, although negatively. Whereas existing research implicitly assumes causality to point in one direction, this study ex-ante allows for a simultaneous relationship. To deal with theexpected endogeneity problem, instrumental variables techniques will be employed. Results show that mobile inventors are more than four times as productive as non-movers. Whereas mobility increases productivity, an increase in productivity decreases the number of moves.