An important inference of the theoretical literature on the development of general purpose technologies is that public investment in their development is necessary if economic growth is to be sustained. The theoretical results are broadly consistent with the empirical generalization that the public sector, particularly military and defense related research, technology development and procurement, has played an important role in the development of most of the general purpose technologies in which the United States is presently globally competitive. These sources are, however, unlikely to play such an important role in the development of new general purpose technologies in the immediate future. Nor is the private sector, burdened by impatient capital, likely to become an important source of new general purpose technology.