This study examined Junior Secondary teachers’ perceived understanding of formative assessment and how it influenced teachers’ pedagogical choices and practices in the classroom. The study adopted a qualitative approach to research using interpretive phenomenological and analytical approaches to provide in-depth insight into the ways in which Junior Secondary teachers perceive, understand and apply formative assessment in their classrooms and how these practices impact student engagement with learning. Three key questions steered the research; 1) What are Junior Secondary teachers’ perceived understandings of formative assessment? 2) What are Junior Secondary teachers’ classroom practices of formative assessment? 3) How do Junior Secondary teachers’ perceptions of formative assessment influence their practices in the classroom in relation to positive student learning outcomes? Data were collected from three Junior Secondary teachers from one school through semi-structured individual interviews, focus group discussions and classroom observations. The data were analysed using Smith, Jarman & Osborn (1999) Interpretive Phenomenological Analysis (IPA) framework and Wiliam and Thompson’s (1998) formative assessment model. Five themes were identified in this study; first, the study found that Junior Secondary teachers’ understanding of formative assessment is evolving and consequently their formative assessment practices were also developing. Second, teachers need to effectively embed formative assessment practices in their lessons to ensure the success of all students. Third, varying student levels meant that there is critical need to differentiate learning to meet individual students’ learning needs. Fourth, there is a need to develop common and shared expectations between systemic, school and classroom approaches to formative assessment and evaluation of students’ learning. Finally, time was an underlying theme that hindered the effective and consistent implementation of formative assessment practices.