This paper tests the presence of technological spillovers for the French case and studies why they occur. Based on a knowledge production function, spillovers are introduced as an external stock of knowledge. Two dimensions are improved: A geographical and a technological effect. The results indicate that technological externalities occur. Spillovers are conditional to technological proximity and, to a lesser extent, to geographical distance. However, externalities are not as generalized as they could be. They do not stem from the whole stock of external knowledge. They flow only through human capital. People thus appear as an essential channel for the diffusion of knowledge.