The purpose of this study was to pursue an enhanced understanding of teacher assessment literacy by investigating tertiary EFL practitioners' assessment-related personal theories, conceptions and beliefs. The study was based on sociocultural theory as a theoretical framework and informed by interpretivism philosophical underpinnings. Twelve teachers from three tertiary educational institutions in the Eastern province of Saudi Arabia participated in semi-structured interviews. The data were analysed employing a thematic analysis approach. The findings revealed diversity, complexity and uncertainty in teacher beliefs and personal theories related to various aspects of assessment and testing. The findings also provided deeper insights into the role of contextual and institutional dynamics that influence teachers' assessment-related decision-making process. These findings have implications for teacher education and professional development programmes in terms of assessment policy, procedures and practice.