This paper provides first evidence on the anatomy of human capital externalitiesarising from both firm-level and regional human capital. Using panel data from German social security records, both at an individual and aggregated at the plant and regional level, I estimate earnings functions incorporating measures of regional and firm-level human capital while controlling for various types of unobserved heterogeneity. The results suggest that the firm-level share of high-skilled workers generates positive, although small social returns to education for low-skilled and skilled workers but not for the high-skilled. This finding is in line with learning based theories of human capital externalities. Some estimates also suggest negative social returns for the regional shares of low-skilled workers. No such effects are found for the firm-level shares of low-skilled workers and the regional shares of high-skilled workers.