This dissertation presents a study on Grade 10 learners’ understanding of key Mathematics concepts in selected secondary schools in Chibombo District of Zambia. The performance of learners in Mathematics countrywide has not been gratifying. This study therefore sought to investigate the Grade 10 learners’ understanding of key Mathematics concepts. This poor performance in Mathematics by learners in many schools at grade 12 final examination in Chibombo District prompted the researcher to conduct this study. The target population was 250 Grade 10 learners in five secondary schools in Chibombo District. The data were collected by means of questionnaire and focus group discussions. A concurrent triangulation design was adopted for the study in which the researcher simultaneously gathered both quantitative and qualitative data merged it using both quantitative and qualitative data analysis. The study revealed that Grade 10 learners’ understanding of the key Mathematics concepts of zero, equal, function, and variable is unsatisfactory. However, it is surprising how these learners can reach secondary school level with such a weak understanding of mathematical concepts, which are ubiquitous in the subject. It is anticipated that the findings of this study will provide curriculum developers with new insights into this problem so that they come up with a curriculum that focuses on learners’ understanding of basic concepts that serve as building blocks to next levels. The Ministry of General Education should rigorously monitor this process and provide both technical and financial support required. Finally, based on the findings of this study, the researcher gave some recommendations for improving learners’ understanding of mathematical concepts in schools. / Mathematics Education / M. Ed. (Mathematics Education)