This chapter presents the various treatments offered by QGIS for network analysis and routing. After a quick definition of networks and graphs, it discusses the rules for construction and verification of networks using GIS. The chapter focuses on the presentation of examples of hydrographic network development and analysis. It consists of several practical works focused on a hydrographic network analysis and routing as part of a global project. The topology of GIS is expressed by a set of rules dealing with relations between spatial entities. In the case of networks, the notion of adjacency or contiguity implies that the spatial units have a common node. The use of a hydrographic networks allow to integrate spatially the physical parameters related to topography, geography, climate and hydrology and to accumulate them downstream and thus provide useful data in the fields of hydrology, geomorphology or ecology.