Peer assessment is an educational arrangement where students judge a peers performance quantitatively and/or qualitatively and which stimulates students to reflect, discuss and collaborate. However, empirical evidence for peer assessment effects on learning is scarce, mostly based on student self-reports or involving comparison of peers and teachers ratings or anecdotal evidence from case studies. Systematic investigation of learning effects necessitates methodological, functional, and conceptual development in peer assessment research. This implies sound (quasi-)experimental studies, the definition of specific peer assessment mechanisms, and affiliations with other research domains. The articles in this special issue address these three needs and offer new directions for research. (http://www.sciencedirect.com/science/article/pii/S0959475209000735)
