In　organic　solar　cells,　the　charge-transfer　(CT)　electronic　states　that　form　at　the　interface　between　the　electron-donor　(D)　and　electron-acceptor　(A)　materials　have　a　crucial　role　in　exciton-dissociation,　charge-separation　and　charge-recombination　processes.　Since　the　introduction　of　active　layers　consisting　of　D-A　bulk　heterojunctions,　CT　states　have　been　the　focus　of　extensive　experimental　and　theoretical　studies.　In　this　Review,　we　assess　the　current　understanding　of　CT　states　and　describe　how　factors　such　as　the　geometry　of　the　D-A　interface,　electronic　polarization　and　the　extent　of　electron　delocalization　affect　their　nature　and　influence　the　radiative　and　non-radiative　decay　processes.　We　focus　on　the　description　and　application　of　fundamental　concepts,　which　provides　the　framework　to　discuss　the　path　to　organic　solar　cells　with　efficiencies　comparable　to　those　in　inorganic　photovoltaic　technologies.