Why does exercise make me put on weight?

I find that when I do a calorie controlled diet I lose weight but it doesnt stay off for long. However, when I try to combine a diet and exercise I always put on weight. I thought this may happen after a period of time due to gaining muscle but I dont seem to tone up either, I simply put on weight, could someone please tell me what I'm doing wrong?
Thanks

When you exercise you probable get more hungry. I am starving when I ride my bike a lot and it is easy for me to justify to myself 'I can eat that because I rode my bike today'.

You exercise, you build muscle. Muscle weighs more than fat. You should focus more on inches, rather than what the scale says.

calories are good fats cholestrerol etc is bad and your probably cutting out carbs then eating them again which is why you gain all of your weight back

Your question is too vague and ambiguous to really answer.

You can put on weight for a variety of reasons. Eating habits, water retention, muscle growth, time of day... also, without knowing what your exercise or diet is, its impossible to correct you on them, since I cant know if you're doing them right or not.