Chapter 4 starts by looking at the concept of reinforcement. Two uses of the term are described: (i) a procedure that results in a change in behaviour, without the theorist making a commitment to any underlying process; (ii) a suggested process of change within the animal underlying changes observed in behaviour, strengthening of a stimulus–response connection. The power of an incentive to draw an animal towards it depends upon (i) its intrinsic properties, (ii) internal states such as nutrient levels, (iii) any associations with the incentive, and (iv) any rival incentives present. The theories of Robert Bolles and Dalbir Bindra are described. The chapter introduces the brain’s dopaminergic pathways, with a focus upon the role of the pathway from the ventral tegmental area (VTA) to the nucleus accumbens (NAcc). The responsivity of this pathway to uncertain reward and ratio schedules is discussed in the context of incentive salience. The chapter concludes with a discussion of two ‘eccentricities’ of behaviour: the misbehaviour of organisms and autoshaping.