Strong assessment knowledge and competency in using assessment is a key professional requirement for teachers. This doctoral study examines the pedagogical processes that bring about teacher learning in assessment during asynchronous online professional development. The study also focusses upon the impact of an asynchronous online learning environment on teacher learning in assessment. Teacher learning is a complex phenomenon and this study is framed by the theoretical perspectives of complexity theory (Opfer & Pedder, 2011) and critical realism (Bhaskar, 1978, 1997). Complexity theory (Opfer & Pedder, 2011) takes the stance that teacher learning involves multiple interactions influenced by factors from the teacher’s personal domain, the school environment and the wider societal and policy spheres. Critical realism holds that causative mechanisms result in observed outcomes, such as teacher learning. The study is also underpinned by the explanatory case study approach, specifically causal process tracing (Blatter & Haverland, 2014) in order to identify which combination of conditions make an outcome possible and which underlying mechanism effectively create an outcome. Data were gathered during a five-module asynchronous online professional development activity. Sources of data included reflective tasks, forum contributions, field notes and participant questionnaires. Follow-up data was also collected when participants returned to school.The study contains reviews of literature in the areas of teacher assessment literacy,assessment theory, teacher professional development, teacher learning and teacher agency and identity. These reviews highlight the pertinent issues from each area. This study is significant as the data is collected during an authentic asynchronous professional development activity in assessment and contributes to the knowledge base both in the pedagogical processes that result in learning and the impact of the asynchronous learning environment on teacher learning. It is also significant as asynchronous online learning is a widely used method of professional development.The study highlights the importance of the process of engaging in quality reflective tasks and the use of specific assessment approaches during asynchronous professional development. In addition, the study explores the role of participant teaching experience in their own learning as teachers. Finally, the study outlines implications for policy, practice and future research in the area of assessment literacy and asynchronous online learning.