The last Nobel Prize in economics was awarded to Richard Thaler for his contributions to Behavioral Economics. Behavioral Economics challenges the standard, neoclassical assumption that economic decision makers are fully rational and only concerned about their own material well-being. Over the last decades new models of human behavior have been developed, often inspired and tested by fascinating economic experiments. In this lecture I will focus on "social preferences", a subfield of behavioral economics to which Thaler and many other economists and psychologists contributed. It seems obvious that many people are not only motivated by their own material well-being but often care about other people. Altruism, spite, inequality aversion, concerns for fairness and reciprocal behavior have an important impact on economic decision making. However, it is often difficult to disentangle the exact motivations that are driving economic behavior. In this lecture I will give a brief introduction into this field and discuss some recent experiments on the dark side of social preferences.