Peer assessment is an arrangement for learners to consider and specify the level, value, or quality of a product or performance of other equal-status learners. Products to be assessed can include writing, oral presentations, portfolios, test performance, or other skilled behaviors. Peer assessment can be summative or formative. A formative view is presented here, in which the intent is to help students help each other plan their learning, identify their strengths and weaknesses, target areas for remedial action, and develop metacognitive and other personal and professional skills. Peer feedback is available in greater volume and with greater immediacy than teacher feedback. A peer assessor with less skill at assessment but more time in which to do it can produce an assessment of equal reliability and validity to that of a teacher. This article describes effective approaches to peer assessment and encourages teachers to incorporate it into their practice.