Patent applications contain citations which are similar to but differentfrom those found in published scientific papers. In particular, patentcitations are governed by legal rules. Moreover, a large fraction ofcitations are made not by the patent inventor, but by a patent examinerduring the application procedure. Using a patent database whichcontains the patent citations, assignees and inventors, we have appliednetwork analysis and built network models. After giving a brief overviewof recent developments in network theory, I will discuss our latest results on patent citation networks.