The literature on institutions and economic growth shows a close relationship between the quality of institutions and prosperity. This paper examines the impact of institutions on investment, and the resulting impact of investment on growth. The private investment rate of countries with better institutional quality is higher, and the productivity of any given level of investment is greater in countries with better institutions. Models that include various indicators of institutional quality along with inputs such as physical and human capital will generally underestimate the impact of institutional quality on growth because they do not account for the indirect impact of institutions on investment, as is done here. The paper also examines the direction of causality to show that higher institutional quality causes more investment, rather than the other way around. Further, future institutional improvements are more likely to occur against a background of poor economic performance than one of sustained growth.