Discusses the relationship between perceived controllability (PCT) of events and optimistic bias (OB) through a research review. The control hypothesis states that the greater the PCT of a negative event, the greater the tendency for people to believe that their own chances are below average. It is important to differentiate the hypothesis that PCT is associated with absolute likelihood from the hypothesis that it is associated with comparative likelihood of a desirable outcome. PCT is not the sole independent predictor of OB. Studies show that comparative optimism is associated with scales of individual differences in generalized perceptions of controllability. Research also shows that the impact of mood on OB may be mediated by the effects of mood on related control beliefs. It is concluded that people are optimistically biased about both negative and positive events they perceive to be controllable.