This paper reviews what we know about the spatial manifestations of knowledge. The knowledge production function addresses the easily measured portion of knowledge produced. Research on learning, particularly interactive and collective learning, in firms and in innovation systems, promises to unveil the human and organizational processes by which knowledge is created, stored, and transmitted to others. Our understanding of innovation and technological change depends on how well we tackle knowledge and its geography. Copyright (c) 2010, Wiley Periodicals, Inc.