This Element traces the evolution of peer assessment in writing instruction and illustrates how peer assessment can be used to promote the teaching and learning of writing in various sociocultural and educational contexts. Specifically, this Element aims to present a critical discussion of the major themes and research findings in the existing studies on peer assessment regarding the three assessment paradigms (assessment of, for, and as learning), and to identify whether and how peer assessment has served the purposes of assessment of, for, and as learning, respectively in writing instruction. This Element highlights the contextual factors that shape the effect of peer assessment in writing instruction and concludes with directions for future research and implications regarding how peer assessment can be successfully used to improve students' writing development.