The focus of this paper is on the effects of the use of the home language (i.e. isiZulu) on teachers' and learners' understanding and use of core concepts in mathematics and science at the senior phase, in contexts where the language of instruction is English. It reports on a national, collaborative, multilingual research project which attempts to address the challenge posed by the National Language Policy of South Africa, namely, that learners have a right to learn through their home language where feasible. The project involved four South African universities and four languages: isiZulu, isiXhosa, Afrikaans and English. The first phase of the research was the identification and translation of core concepts in the science and mathematics curriculum, which culminated in the development of a multilingual resource book. The second phase involved the evaluation of the book with in-service teachers by means of questionnaires, workshops and focus-group interviews for purposes of triangulation. These data were the basis on which teachers for the tracer study were selected. This part of the study involved classroom observation of mathematics and science lessons at intervals over a two-month period. Findings from the KwaZulu-Natal regional data revealed various complexities regarding the use of the home language for communicating scientific and mathematical knowledge. These included unfamiliar terms leading to the question of standardisation and an initial preference for the use of terminology in English with explanations in the home language.