How formative assessment is enacted is not fully understood. This thesis explored formative assessment enactment in Ireland and Scotland with a specific focus on the factors that promote or inhibit enactment. The literature review revealed a gap in the research on how formative assessment is enacted in second level science classrooms, particularly in Ireland. The study employed a multi-site case study methodology with activity theory providing the theoretical lens. A sample of four second level schools (two in Ireland and two in Scotland), four school leaders, and eight science teachers with their lower second level science classes were recruited to the study. Data were gathered using interview, observation, video data, desk research, and the Q methodology. Qualitative data were coded using themes relating to formative assessment (Wiliam and Thompson, 2007) and activity theory (Engeström, 1987). Q data were analysed quantitively using PQ Method software (Schmolck & Atkinson, 2002). Findings from this thesis illustrated that in Ireland there is a strong focus on tools for formative assessment without consideration of its true purpose: improved learning and responsive teaching. While teachers plan to enact formative assessment, they were not using it to inform teaching and learning. The cultural context of high stakes examinations in Ireland was also undermining formative assessment enactment. Scotland had a longer time with formative assessment in their Curriculum for Excellence (Scottish Executive, 2004). However the data showed teachers were still not enacting formative assessment effectively. The findings add new knowledge to the scholarship on formative assessment. Most notably a pair of factors for effective formative assessment enactment are presented: cultural context and assessment literacy. In addition, this thesis uncovered a variety of other factors that were influencing formative assessment enactment including curriculum, whole school initiatives, professional development, school community, teacher professional identity, and teacher student relationships.