This Thematic Study is a joint venture between ICOMOS, the advisory body to UNESCO on cultural sites, and the International Astronomical Union. It presents an overall vision on astronomical heritage, attempts to identify what constitutes "outstanding universal significance to humankind" in relation to astronomy, and identifies broad issues that could arise in the assessment of cultural properties relating to astronomy. This is the first Thematic Study in any field of science heritage. It is elaborated using examples of properties from around the world, including some already on the World Heritage List or national Tentative Lists. The subject matter ranges from early prehistory to modern astrophysics and space heritage, and also prominently includes dark sky issues and modern observatory sites. An e-version of the Thematic Study was published in June 2010 in time to be presented to the 2010 meeting of UNESCO's World Heritage Committee, where it was duly approved. It has been circulated officially by the WHC to all of UNESCO's National Commissions. This full-colour paperback edition with some updates, and reformatted to new ICOMOS standards, was published in 2011 and is now offered for public sale.