This thesis sought to answer the question: what factors help or hinder adult Māori second language learners to become proficient speakers of te reo Māori? I ask this question at a moment in history when te reo, like other indigenous languages, sits on the edge of a precipice in terms of its survival as a living language. Ambivalence and inaction will almost certainly constitute a push towards oblivion. Nothing less than concerted effort can pull te reo back from the brink. But what kind of concerted effort is needed and how can individual learners optimise their efforts to learn te reo? Te reo must survive because, as Sir James Henare put it, ‘ko te reo te hā o te Māoritanga’, meaning ‘the language is the essence of Māori culture.’ Without te reo the very survival of Māori people as Māori is in doubt. My questions and the thesis occupy a multidisciplinary space, drawing on research from adult second language acquisition, teaching pedagogy and language revitalisation. The thesis makes an original contribution to all three bodies of literature by examining data from a previously untapped source; successful adult Māori language learners. I interviewed 17 participants. All highly proficient second language speakers of te reo. All learned te reo as adults. Adult second language learning of endangered languages is broadly acknowledged in Second Language Acquisition (SLA) as essential to re-establishing Intergenerational Transmission of Language (ITL) in the home to ensure an endangered language is passed on to future generations. The interview questionnaire was based loosely on 10 help/hinder factors I identified through a review of the literature from the three research fields (adult second language acquisition, teaching pedagogy and language revitalisation). The ten factors identified in the review were; language aptitude, age, learner attitudes and motivation, learning strategies, instruction, agency and anxiety, wairua (the spiritual dimension), demography, language status, and language planning.