Education is an important aspect of a country's development process. It was with this in mind that the EFA concept, part of the MDGs, was adopted by most African countries, including the Republic of Congo. When this program was assessed at mid-term, the high gross enrolment ratios seemed to mask the shortcomings of the internal situation of the Congolese education system. In fact, the proportion of children who were behind in their schooling was 75% in 2005, 43% in 2012 and 36% in 2015. This situation generates additional costs for parents, and indirectly affects the country's socio-economic progress (Haramein, 1970). These reasons led to the following research questions: What are the explanatory factors of school backwardness among primary school children in the Republic of Congo? Are they the same for boys and girls? The general aim of this study is therefore to contribute to the improvement of knowledge about school backwardness in the Congo, in order to help decision-makers take action to improve the internal performance of the Congolese education system on the basis of scientific evidence.