In this article open access is defined, and the resources and issues of greatest relevance to the medical librarian are discussed. The economics of open access publishing is examined from the point of view of the university library. Open access resources, both journals and articles in repositories, are already significant and growing rapidly. There are close to 2300 fully open-access peer review journals listed in the Directory of Open Access Journals (DOAJ) (320 health sciences titles are included). DOAJ is adding titles at a rate of 1.5 per day. An OAIster search of resources in repositories includes more than 7.6 million items (a rough estimate of the number of articles in repositories, although not all items are full text), and this number will exceed one billion items before the end of 2007. Medical research funders, including the US National Institutes of Health, the Wellcome Trust, the UK Medical Research Council, and the Canadian Institutes of Health Research, either have implemented or are considering open access policies. This will drive greater growth in open access resources, particularly in the area of medicine. There are implications and leadership opportunities for librarians in the open access environment.