Recent developments on photovoltaic elements based on solid state composites of conjugated, semiconducting polymers mixed with buckminsterfullerene are reviewed. The photoinduced charge transfer from donor-type semiconducting conjugated polymers onto acceptor-type conjugated polymers or acceptor molecules such as buckminsterfullerene is reversible, ultrafast (within 100 fs) with a quantum efficiency approaching unity, and the charge separated state is metastable (up to ms at 80 K). This phenomenon of photoinduced electron transfer leads to a number of potentially interesting applications which include, among others, sensitization of the photoconductivity, reverse saturable absorption (optical limiting), and photovoltaic phenomena. Recent studies on the realization of photovoltaic elements with 3% power conversion efficiency are reported.