The major theories of consciousness that distinguish conscious from unconscious states can be grouped into two main classes, either higher-order or integration theories. There is evidence that different types of mental states can be unconscious, though that conclusion depends on the theory of consciousness assumed. Unconscious memory (in the sense of the influence of a prior event not recollected) can shape perception and liking and control our behavior. Subliminal perception can produce semantic priming and guide attention and decision making; and optical variables that a person describes incorrectly can guide action. Implicit learning can shape judgments and choices in complex environments. Unconscious intentions can allow people to respond appropriately in goal-directed ways while the person experiences the actions as involuntary. Unconscious attitudes are no more or less plausible than any other mental state being unconscious, but it has been hard to obtain evidence for unconscious attitudes as distinct from gut reactions one does not agree with.