The　concept　of　functional　dependencies　plays　an　important　role　in　relational　databases　where　it　is　used　in　integrity　enforcement　and　database　design.　The　corresponding　concept,　functional　dependencies　in　XML　(XFDs),　has　been　widely　investigated　over　the　past　few　years.　Although　some　proposals　have　been　made　and　most　of　them　are　based　on　path　that　is　a　crucial　factor　for　the　expressive　power　of　XFDs,　there　seems　to　be　no　consensus　on　how　to　specify　them.　In　this　paper,　we　survey　the　previous　work　in　terms　of　the　approaches　they　employ.　Some　related　issues　are　discussed.　In　particular,　we　analyze　the　effects　of　null　values　and　how　to　compare　element　nodes　with　complex　type.　We　also　discuss　the　logical　implication　and　normalization　for　XML　in　brief.　A　set　of　requirements　for　a　good　XFDs　definition　are　concluded.　©　2010　IEEE.