This paper reports on the results of a large infrastructure investment experiment in which $68 million in spending was randomly allocated across a set of low-income urban neighborhoods in Mexico. We show that the program resulted in substantial improvements in access to infrastructure and increases in private investment in housing. While a pre-committed index of social capital did not improve, we find an apparent decrease in the incidence of personal assault and teen misbehavior in neighborhoods where investments were made. The program increased the aggregate real estate value in program neighborhoods by two dollars for every dollar invested. (JEL H76, O18, R23, R31, R53, Z13)