PostGIS (http://www.postgis.net) is, without question, the most popular open source spatial database platform available today. PostGIS is not actually a database in itself; it is in fact an open source library that spatially enables the open source PostgreSQL database. However, the name is so frequently used when referring to a PostgreSQL database with the extension installed, that it is the term we will adopt when discussing it in this book. In the open source geospatial world, GeoServer and PostGIS are perhaps the most common combination of spatial database and web mapping server. It is the database that GeoServer supports out of the box, without the need for any further configuration.