Professional learning for teachers is an area of international interest, not least because teacher quality is seen to be a factor in pupil outcomes. Scholars agree that immersion teaching requires a particular knowledge and skill set; yet there is a paucity of international research on the professional learning of bilingual and immersion teachers and none in the context of Gaelic-medium education. This thesis examines what professional learning opportunities Gaelic-medium primary teachers perceive they require to support them in teaching Gaelic as a language and teaching the curriculum through the medium of Gaelic. Factors such as stage of career, teacher language background and contexts for teacher learning were also considered. Theories of bilingual and immersion education, second language learning and teacher learning informed the study. Semi-structured interviews were conducted in Gaelic with 25 Gaelic-medium primary teachers, including teaching managers, across 7 local authorities. The teachers completed a short questionnaire eliciting relevant information of educational background, professional learning and experience that supplemented the interview data. Thematic analysis was used to identify key professional learning themes across the transcripts. The results show that professional learning to further develop teacher language proficiency, including a deeper understanding of the grammatical structures of Gaelic, was of key importance to the teachers. The research also demonstrated that the teachers perceived a better understanding of bilingual and immersion education, how to teach language in the curriculum and extending their pedagogical practices were necessary areas of further learning for them. There was little Gaelic-specific professional learning available to the teachers, with English-medium local and national priorities taking precedence. The role of leaders and managers in choosing and facilitating professional learning opportunities was identified as a significant contextual factor shaping teacher learning. While there is a scarcity of bilingual and immersion studies in teacher professional learning, there is a wealth of research related to the language areas identified by the teachers for further learning which are drawn on in discussion. This thesis offers a contribution to the professional learning literature base of Gaelic-medium teachers, based on their needs, through its analysis of teacher-identified learning opportunities in relation to teaching Gaelic as a language and teaching the curriculum through Gaelic. Key areas of teacher language proficiency, form-focused instruction, immersion pedagogies and assessment are identified for professional learning across the career continuum to further enhance classroom practice. It is recommended that professional learning opportunities in these teacher-identified areas be progressed at both national and local level, through flexible methods of delivery, to ensure availability and accessibility to the geographically dispersed Gaelic-medium teacher group.