Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. Open Data is a philosophy and practice requiring that certain data are freely available to everyone, without restrictions from copyright, patents or other mechanisms of control. It has a similar ethos to a number of other "Open" movements and communities such as open source and open access. However these are not logically linked and many combinations of practice are found. The practice and ideology itself is well established (for example in the Mertonian tradition of science) but the term "Open Data" itself is recent. Much of the emphasis in this entry is on data from scientific research and from the data-driven web. In some cases Open Data may be considered as more properly Open Metadata and there is not yet a consistent formalisation. This article uses recent publications and activities to define the scope of the concept and term.