Geobacter bacteria are the only microorganisms known to produce conductive appendages or pili to electronically connect cells to extracellular electron acceptors such as iron oxide minerals and uranium. The conductive pili also promote cell-cell aggregation and the formation of electroactive biofilms. The hallmark of these electroactive biofilms is electronic heterogeneity, mediated by coordinated interactions between the conductive pili and matrix-associated cytochromes. Collectively, the matrix-associated electron carriers discharge respiratory electrons from cells in multilayered biofilms to electron-accepting surfaces such as iron oxide coatings and electrodes poised at a metabolically oxidizable potential. The presence of pilus nanowires in the electroactive biofilms also promotes the immobilization and reduction of soluble metals, even when present at toxic concentrations. This review summarizes current knowledge about the composition of the electroactive biofilm matrix and the mechanisms that allow the wired Geobacter biofilms to generate electrical currents and participate in metal redox transformations.