This book is about open data, i.e. data that does not have any barriers in the (re)use. Open data aims to optimize access, sharing and using data from a technical, legal, financial, and intellectual perspective. Data increasingly determines the way people live their lives today. Nowadays, we cannot imagine a life without real-time traffic information about our route to work, information of the daily news or information about the local weather. At the same time, citizens themselves now are constantly generating and sharing data and information via many different devices and social media systems. Especially for governments, collection, management, exchange, and use of data and information have always been key tasks, since data is both the primary input to and output of government activities. Also for businesses, non-profit organizations, researchers and various other actors, data and information are essential.