The present paper can be considered as divided in two parts: in the first one, we provide a review of the methods of complex networks that have been mainly used in the applications to the analysis of financial data. We focus on the following topics: the usage of the correlation matrix, systemic risk, integrated ownership and control, board of directors, interbank networks, and mutual funds holdings structure. The second part shows this last subject and provides new analyses. The main findings outline that there are substantial differences in geographical allocation among the different European fund managers. Five larger European countries dominate the market of mutual funds. The belonging of UK and Swiss opt-outs of the eurozone could be a probable explanation for our results on community detection, that give a snapshot of a sort of ``geographical organization'' of the core of mutual funds portfolios.