This study presents a glimpse into the private classrooms of biology instructors and the way they practice formative assessments within a college context. Drawing on the personal practice assessment theory model from Box, Skoog and Dabbs (2015), we carried out a multiple case study to investigate two biology instructors' theories in enacting formative assessment practices. Data collected included classroom observations, instructor interviews, course artifacts, and student focus groups. Qualitative data analysis revealed that each instructor's core personal practical assessment theories affected implementation of formative assessment. Tasha's core assessment theories led her to believe that assessment should be a carefully planned motivational and learning opportunity for students. Meanwhile, Jack viewed assessment as a diverse and stress-free student learning experiment. Cross-case analysis revealed that the teachers' reasoning and decision-making differences were based on the interaction of their personal practical assessment theories and contextual elements. Overall, this study provides insights into the practice of formative assessment in higher education and identifies some challenges and opportunities such assessment presents.