The focus of this thesis was the hedonic value of stimuli, which is more commonly known as pleasure or positive affect. First, the scientific meaning of hedonic value was dissected. Second, a classification identifying core causes of positive affect was created. The classification was derived from specific positive moments reported by individuals throughout a day (collected through experience sampling methodology). Seventeen triggers of positive affect were identified, which were extracted from the data rather than originating from theory. Third, affective influences on reflexive-like motor responses were investigated using an approach-avoidance task. Contrary to previous studies, approach reaction times were not speeded by highly affective stimuli. Instead, a novel non-emotional effect was found on reaction times, which could directly explain the current results, and those of previous studies, in non-affective terms. Fourth, the propagation of hedonic reactivity from pleasurable to neutral stimuli was investigated. Contrary to expectations, the evaluative conditioning procedure utilised did not exhibit a phenomenon called blocking. Instead, 'liking' spread non-selectively to all stimuli co-occurring with the source hedonic stimulus. Fifth, the positive effect of pleasure on goal-directed motivation was established: participants were found to press a food trigger harder for highly palatable snacks compared to bland snacks, even though participants were not informed about the hidden measurement of forces. Additionally, the impact of hedonic value on actual food intake was quantified with best-fit equations that predicted consumption at both the group and individual level. In the last study, hedonic habituation, or the inhibitory effect of pleasure on itself, was demonstrated: eating pleasant snacks, as compared to bland ones, reduced the hedonic ratings of test foods that were consumed afterwards. Finally, these inputs and outputs of hedonics were integrated into a model specifying principal roles of pleasure in human behaviour. This pleasure-incentive model explains the effects of pleasure on incentive motivation, and makes important predictions about the mechanisms of pathological conditions such as over-eating and drug addiction.