The brain is an organ of decision making. In order to choose actions that will maximize the chance of survival and reproduction, the brain has to recognize the animal's environment and choose appropriate actions within it. Traditionally, formal theories and cognitive mechanisms of decision making have been studied largely separately in economics and psychology. Neuroeconomics embraces both of these disciplines and unites them with neurobiological studies of brain's reward system involved in outcome monitoring and action selection. Non-invasive neuroimaging studies can investigate the brain activity of humans during complex financial and social decision making. Results from these studies often find practical applications such as neuro-marketing. In addition, animal studies advance our basic knowledge of the brain's reward system. By elucidating the biological basis of our values and preferences, neuroeconomics provides unique insights into many fundamental questions on human behavior.