This study proposes to investigate the threshold effects in the productivity of infrastructure investment in developing countries. It concludes to their presence in the relationship between output and private and public inputs as well as network effects in the productivity of infrastructure. When the available stock of infrastructure is low, investment has the same productivity as non-infrastructure investment. On the contrary, when a minimum network is available, the marginal productivity of infrastructure investment is greater than the productivity of other investments. Finally, when the main network is achieved, its marginal productivity becomes similar to the productivity of other investment.