The performance of Mathematics at Kenya Certificate of Primary Education has remained low compared to other subjects at this level (KNEC, 2006,2007,2008). This study investigated the factors affecting the teaching and learning of Mathematics in selected public primary schools in Usian Gishu between the period of May and July 2010. The study was guided by the following objectives: to establish the teacher, learner, syllabus and school-related factors affecting the teaching and learning of Mathematics and to determine challenges faced in Mathematics teaching and learning in primary schools. The study employed a descriptive survey study design and was guided by the education production function theory. The study population comprised of head teachers, teachers and pupils in public primary schools in Usian Gishu of Rift-Valley province. Stratified random sampling was used in selecting 15 schools for participation in this study. Purposive sampling was used to select fifteen head teachers and 45 teachers of Mathematics for inclusion in this study.