This contribution starts from the assumption that volunteered geographic information is a technological, cultural and scientific innovation. It therefore offers first some general background on the context that has fuelled the development of VGI and the lively scientific debates that have accompanied its success. The paper then focuses on the nature of this data by describing the main elements of VGI: the geographical reference (coordinates, geotag, etc.), the contents (texts, images, etc.) and the producers’ profiles. The opportunities and the criticalities offered by this data are described with examples drawn from recent literature and applications to highlight both the research challenges and the current state of the subject. The chapter aims to provide a guide to and a reference picture of this rapidly evolving subject.