There is much evidence that naturally occurring behaviors (e.g., the ingestion of food and water) and social behaviors (e.g., play, maternal behavior) can induce a reward state. This review includes definitions to distinguish between “reward” and “reinforcement,” and a description of methods to assess reward and demonstrate that social interactions can indeed produce a positive affective (PA) state. Operant responses, partner preference, and sexual incentive motivation are all effective methods for evaluating approach behaviors under different conditions. The method most frequently used to evaluate a positive affective or reward state is conditioned place preference (CPP), which entails modification of an animal's initial preference after alternating exposure to a control stimulus in one chamber and a rewarding condition in the other. At the end of the training the animal shows a clear preference for the compartment associated with the rewarding stimulus. CPP demonstrates that it is possible to use different treatments and naturally occurring behaviors (e.g., water or food consumption, exercise) to induce a reward state. Sexual interactions and other social behaviors also produce a clear change of preference, indicating the induction of a reward or PA state. The reward state in males and females is mediated by opioids, and the medial preoptic area of the anterior hypothalamus is a crucial site for sexual reward.