This thesis investigates the development of a lexicon and register to facilitate the teaching of mathematics in the medium of te reo Māori. This sociolinguistic development is situated within the wider Māori language revitalisation movement in Aotearoa/New Zealand, of which Māori-medium education is a central component. Lexication of a language is a subdomain of corpus planning. Corpus, status and language-in-education planning are considered the three major interconnected domains of language planning and policy (LPP). For decades, the primary focus of LPP research has been on the activities of states and their agencies at the macro (national) level. The significance of this study is that it addresses this gap in the research, by investigating the development of LPP at the meso/micro-level—in this case, with respect to the development of a mathematics register for an endangered indigenous language, te reo Māori. The thesis draws on two major participant data sources. The first is the narratives of key individuals at the forefront of lexical development for Māori-medium mathematics who recounted their experiences. The second is interviews with principals and teachers from two Māori-medium case study schools to examine their own learning and usage of the standardised pāngarau lexicon. Two complementary theoretical frameworks were used to investigate the topic. First, an LPP theoretical framework was applied to investigate the complex relationships between the various levels of LPP that have impacted on the evolution of te reo Māori and Māori-medium schooling. Second, a linguistic framework, systemic functional linguistics (SFL), was used to investigate the linguistic and pedagogical issues. Thesis findings show that the status of te reo Māori was variously affected by linguistic ideologies such as assimilation, which in turn led to such policies as English only in schooling. As a consequence, at the micro-level of LPP, Māori-medium mathematics development went into a hiatus for over 100 years, and at the macro-level, considerable language shift occurred to English, causing te reo Maori to become endangered. In response to language revitalisation efforts, the (re) vernacularisation of te reo Māori over the past 30 years has exhibited a complex combination of both bottom-up and top-down LPP. Within this, early pāngarau lexical developments were largely driven by local schools and their communities, which sometimes centred on the maintenance of their tribal dialects, often in conflict with later LPP agency language goals. Subsequently, lexication for pāngarau was influenced by macro-level sociolinguistic beliefs such as linguistic purism, which in turn have influenced the technical approach to creating terms and may have created particular pedagogical issues, such as a lack of intertranslatability between te reo Māori and English at the higher levels of mathematics study. While Māori-medium schooling has contributed significantly to language revitalisation in the past 30 years, its development has been very ad hoc. In the absence of a “formal” language plan for Māori-medium education, policy has been significantly driven by national education (English-medium) imperatives since the late 1970s to the present day. On the other hand, these policies have favoured mathematics education and directly supported the development of the Māori-medium specialised lexicon and register.