PEER (Publishing and the Ecology of European Research) is an EC supported collaboration between publishers, repositories and the research community aimed at improving understanding of the effects of the large scale deposit of stage two (accepted) manuscripts in open access repositories (Green Open Access). Through the creation of an observatory with European content from approximately 300 peer-reviewed journals from participating publishers, PEER aims to monitor the effects of systematic archiving over time. Research commissioned from qualified independent teams addressing author and reader behaviour, article usage at repository and publisher sites and the economics of publisher assisted deposit and author self archiving, will result in a number of outcomes including: evidence based guidance for the evolution of open access policy; a model of the effects of archiving on the traditional publishing systems and foster trust and mutual understanding between publishers and the research community for the overall benefit of European research.