The talk gives an overview on building web GIS solutions using open source geospatial software components. A selection of the most advanced and popular open source components, their roles and functionalities in an application framework are described. A complete stack for building web GIS solutions can include a client-server application framework, a web mapping engine supporting cartographic rendering functionality and basic GIS capabilities, data storage solutions such as spatial databases, and additional tools extending or enhancing the basic GIS capabilities of the mapping engine. The concept of an interoperable software solution is illustrated by a variety of examples. Web GIS application frameworks are either complete client-server solutions such as Mapbender or Web GIS clients such as Open Layers or Leaflet. Internet mapping engines such as MapServer and GeoServer render images, provide basic GIS functionality and/or provide the functionality for serving Web mapping services (WMS) and Web feature Services (WFS). Using cached and tiled data in a web GIS application can substantially speed up its response time. For example TileCache is a server and caching solution that can be used for the creation of cached data from WMS. Spatial databases include solutions such as PostGIS and MySQL. PostGIS itself provides comprehensive GIS capabilities that can greatly extend the mapping engine’s capabilities and simplify execution of geo-processing tasks using spatial SQL. Additional tools can be integrated with the framework to provide specific GIS capabilities.