Throughout the education system, assessment is a multi-faceted concept used in various forms and for various purposes such as accountability and improvement (OECD, 2012). Post-Primary education in Ireland has experienced significant changes to the curriculum and with that assessment reform. This is evident in lower secondary, where we are now in steady state at junior cycle since the introduction of the Framework in 2015, and in the upper secondary, where senior cycle reform is underway with a number of new specifications fully published within the system. Central to this change is the development of a learning outcome approach to teaching, learning, and assessment, which places the learner at the centre of the assessment process facilitated by the teacher. Learner voice and the rights of the learners are at the centre of teachers' practice. These recent changes have provided a shift in the paradigm in relation to teaching, learning, and assessment across education, as the approach to assessment places greater emphasis on the learner at the centre of this process. The development of assessment knowledge and competency, known as teacher’s assessment literacy, is a challenge for teachers particularly in the area of how to use assessment, and this challenge is central to their beliefs and cultures. This doctoral study examines a case study of teacher beliefs and school culture in developing assessment literacy in Irish Post-Primary Schools. The study is conducted through the lens of a case study analysis with semi-structured interviews in a post-primary school. The aim of the research is to identify the role teachers’ beliefs and cultures play in developing assessment literacy. Data was gathered across a number of semi-structured interviews and an engagement with the teaching and learning group of the school across a six-month period. This study was conducted during the school academic year 2021-2022. It will assist in contributing to the research in Ireland relating to teachers’ beliefs and culture relating to assessment. This research captures the importance of engaging with schools directly, to provide valid and meaningful evidence relating to teachers practice over a period of time. In this instance, it is related to teachers’ assessment literacy and the role played by beliefs and cultures in developing this practice. By engaging with the school on a one-to-one basis, the researcher was able to address the challenges and concerns that the teachers were facing in the here and now, relating to curriculum change with a specific focus on the development of their assessment literacy. Additionally, the study examines the role played by continuing professional development (CPD), the continuum of education, teachers’ assessment literacy and the role that the participants experience in teaching has on their own learning as teachers. The study concludes by outlining the implications for the system and future research in the area of assessment literacy relating to teachers' beliefs and culture.