Starting from the seminal work of Dewey, Piaget, Vygotsky, and Bruner, peers have been conceptualized as potential mediators in students’ learning and development. In recent decades, there has been an increasing interest in how students’ learning can be fostered through involving them in assessment via self- and peer assessment. Both self- and peer assessment are now well-established fields of research with the following main lines of work: (a) the reliability/validity of self- and peer assessment scores; and (b) the effects of such assessment on students’ learning, which have mostly been studied after the emergence of research on formative assessment. Two more topics that have recently received more interest are: (c) the effects of self- and peer assessment on self-regulated learning and metacognition and (d) the role of psychological and social factors in self- and peer assessment. Consequently, researchers have become interested in the type of information that students exchange in peer assessment situations, with research focused on whether the quality of such information can lead to improved learning compared to just providing a score (i.e., peer scoring). This information is known as peer feedback. There has been an increased interest in peer feedback – a trend reflected in the publication of dissertations that focus on various aspects of peer feedback. The aim of this chapter is to explore the concept of peer feedback, presenting the results of the main dissertations and discussing the key empirical themes that have been investigated.