The amount of available data is increasing very fast. With this data the desire for data mining is also growing. More and larger databases have to be searched to find interesting (and frequent) elements and connections between them. Most often, the data of interest is very complex. It is common to model complex data with the help of graphs consisting of nodes and edges that often are labeled to store additional information. Applications can be found in very different fields. For example, the two-dimensional structure of molecules often is modeled as graphs having the atoms as nodes and bonds as edges. The same holds for DNA or proteins. Web pages and links between Web pages also can be represented as graph. Other examples are social networks as citation networks and CAD circuits; graphs can be found in a lot of different application areas.