This study analyzes the impact of increasing population density in Kenya’s rural areas on smallholder behavior and welfare indicators. We first present evidence to explain how land constraints can be emerging within an overall context of apparent land under-utilization. Using data from five panel surveys on 1,146 small-scale farms over the 1997-2010 period, we use econometric techniques to determine how increasing rural population density is affecting farm household behavior and livelihoods. We find that farm productivity and incomes tend to rise with population density up to 600-650 persons per km2; beyond this threshold, rising population density is associated with sharp declines in farm productivity, total household income, and asset wealth. Currently 14% of Kenya’s rural population resides in areas exceeding this population density. The study concludes by exploring the nature of institutional and policy reforms needed to address these development problems.