Two no nos:
#1 - you asked a silly question which a simple search ANYWHERE could have answered.
#2 You bumped your own post 4 minutes after posting it.
Its caused by estrogen look at old guys or fat guys, see tits? Thats gyno most of the time, BB'ers get it in different degrees at different dosages (very individual) if they dont have anti-estros. Some can get it off of 100mg of deca others cant get it at over a gram of test in a huge stack. Most people might feel it threatining somewhere between 500-750 mg of test.
search next time.