The Kenya government implemented alternative provision of basic education and training (APBE&T) policy in 2011 to support non-formal primary schools in providing basic education to children not able to enroll in public formal primary schools. A case study of Mathare Constituency in Nairobi, Kenya was carried out to find out determinants of access and participation in these schools. Primary-school-age children not enrolled in either formal primary schools or in the non-formal primary schools fail to attain basic education. Since non-formal primary schools are instituted in Kenya to provide basic education to children who cannot access formal primary schools, they should be accessible to all kinds of children. This paper reports findings on the determinants of access and participation in non formal primary schools in Mathare Constituency in Kenya. The implication of the study is that to increase access and participation in non-formal primary schools in Kenya, the Ministry of Education Science and Technology should provide funding to non-formal primary schools, establish a school-feeding programme and provide for learning environments that conform to its set standards