HSO launched in September 2008 as a virtual learning center with browse and search functions. HSO provides free, online linkages to a comprehensive collection of top-quality courses and references in medicine, public health, pharmacy, dentistry, nursing, basic sciences, and other health sciences disciplines. These materials are donated, hosted, and maintained by our distinguished content partners, so quality is maintained, and materials can be constantly updated. HSO is a sieve that includes world-class materials (currently numbering >50,000 resources), hand-selected by clinicians and other experts from already-existing reliable sources and resource collections. This includes medical specialty societies, accredited continuing education organizations, governments, and top-ranked universities such as Columbia, Harvard, Johns Hopkins, MIT, Penn, and UCSF.