Article published in the Journal of Development Effectiveness, Volume 8, 2016 - Issue 3. Published Abstract: To improve learning outcomes, an intervention in Kenya called the Primary Math and Reading (PRIMR) Initiative provided pupil learning materials, teachers’ guides and modest teacher professional development in mathematics. This paper presents the causal impact of PRIMR’s mathematics intervention on pupil achievement indices for procedural and conceptual numeracy, using a differences-in-differences analytic strategy. The mathematics intervention produced modest, statistically significant results: generally similar results for males and females, a larger impact in grade 2 than grade 1, a larger impact in nongovernment schools than public schools, and smaller outcomes in mathematics than for English or Kiswahili. These findings have relevant policy implications in Kenya given an impending national mathematics programme.