Over the past two decades, formative peer assessment has become a popular instructional approach. Initially, it was more readily applied in higher education but has since expanded to other educational levels, including primary and secondary education. The popularity is understandable given the increased amount of feedback by multiple peers and enhanced awareness of performance criteria. Although it is increasingly acknowledged by the research community that formative peer assessment is inherently a social endeavour, the collaborative nature is simultaneously the least-explored mechanism. The contributions in this special issue address this gap conceptualising peer assessment and peer feedback as both an individual and a collaborative learning practice. Furthermore, we highlight core learning conditions: learner characteristics, domain and task characteristics, and, finally, instructional scaffolds.