Developments in forms of peer learning 1981-2006 are reviewed, focusing mainly on peer tutoring, cooperative learning, and peer assessment. Types and definitions of peer learning are explored, together with questions of implementation integrity and consequent effectiveness and cost-effectiveness. Benefits to helpers are now emphasised at least as much as benefits to those helped. In this previously under-theorised area, an integrated theoretical model of peer learning is now available. Peer learning has been extended in types and forms, in curriculum areas and in contexts of application beyond school. Engagement in helping now often encompasses all community members, including those with special needs. Social and emotional gains now attract as much interest as cognitive gains. Information technology is now often a major component in peer learning, operating in a variety of ways. Embedding and sustainability has improved, but further improvement is needed.