This article contributes new evidence on factors associated with low achievement among pupils in urban informal neighborhoods in Nairobi, Kenya. The authors use three different data sets to examine the effect of residence in particular neighborhoods, pupil gender, primary school type, and household socioeconomic status on pupil achievement in the Kenya Certificate of Primary Education (KCPE) examination results for 2005 and 2006. Results suggest that residence in Nairobi's informal neighborhoods of Korogocho and Viwandani, enrollment in a public school, and one's gender, if female, are the strongest explanatory variables for low achievement. Policy implications from the study results are discussed. (contains 6 tables.)