This case study explored the process of English as a second language (ESL) teachers' participation in online self- and peer assessment (OSA and OPA) during an online teacher professional development (TPD) program focusing on task design and their perception of the subsequent impact on their understanding of tasks. Five in-service ESL teachers working in government-run schools in India participated in the study. Data were collected through a checklist, a rating scale, semistructured interview, videorecording of all OPA sessions, and field notes. The results substantiate the collaborative view of reflection proposed by Mann and Walsh (2013) even though teachers' self- and peer assessment activities happened online. In OSA, first, the participants prepared through self-reflection and planning, then self-evaluated, and finally made corrections. Similarly, in OPA, the participants engaged in dialogue and evaluation before consolidating with peers. The results also indicate that the participants held a positive opinion about the impact of OSA and OPA on their understanding of tasks. Though it was a small-scale study, the findings may have implications for teacher development. OSA and OPA could be used as formative assessment tools in teacher professional development programs, and future researchers may be interested in finding their impact on teachers' knowledge and practice.