INTRODUCTION: Improving evidence informed decision-making in immunisation is a global health priority and many low and middle-income countries have established National Immunisation Technical Advisory Groups (NITAGs) as independent technical advisory bodies for this purpose. NITAG development and strengthening has received financial and technical support over the past decade, but relatively little evaluation. This study examined NITAGs in six low and middle-income countries (i.e. Armenia, Ghana, Indonesia, Nigeria, Senegal, Uganda), to examine functionality, quality of recommendation development, and integration with national decision-making bodies and processes. METHODS: A mixed-method case-series design, used semi-structured interviews, NITAG meeting observations, and document review. Data were analysed thematically. RESULTS: Five NITAGs had been legally established with terms of reference and appeared well functioning, with Ghana's in development. All NITAGs had standard operating procedures and nomination procedures to ensure a range of expertise, generally comprising 10-15 core, 1-5 secretariat, and several ex-officio members. Aside from economics, NITAGs reported a wide range of member expertise. Newer NITAGs had particular concerns about funding. Four used formal conflict of interest procedures, although some commented that implications were not always understood. NITAGs valued local data, and limited evidence suggested NITAG presence might reinforce data production through surveillance and local research studies. All observed meetings demonstrated due process and evidence-based decision-making processes were generally followed, with a critical role played by working-group data syntheses and assessments. NITAGs were seen as well integrated with ministry of health (MoH) decision-making and MoH interviewees were positive about NITAG contributions, indicating NITAGs had an important role. Collaboration with other bodies was more limited, but mitigated by NITAG members' cross-membership in other bodies. CONCLUSIONS: NITAGs have an important and valued role within national immunisation decision-making. However, their position remains insecure, with the need for sustainable technical and financial support.