The　cyclic　deformation　behaviors　of　titanium　and　zirconium　have　been　systemetically　investigated.　The　slip　and　twinning　systems　are　discussed.　The　striking　feature　is　that　twinning　plays　a　crucial　role　in　cyclic　deformation.　Fatigue　damage　is　associated　with　twins,　but　twinning　induced　plasticity　at　low　temperatures　is　also　found.　The　interactions　among　different　types　of　dislocations　and/or　twins　are　discussed.　The　stacking　faults　associated　with　twins　have　been　observed,　although　titanium　is　known　to　have　high　stacking　fault　energies.　It　is　proposed　that　the　complexity　of　fatigue　behavior　of　hcp　metals　stems　from　the　synergetic　effects　of　these　crystal　imperfections.