Decision theory has been dominated by a logic of complexity: The world is complex, and so must be the models used to make decisions. Standard models evaluate as many options as possible, by making tradeoffs between each option’s good and bad features. Problems with this decision theory are well known. It is challenging to gather the information the models require, and decision-makers do not understand the models. In the last three decades, research in cognitive psychology and behavioral ecology has provided the foundations of an alternative decision theory: This theory is inspired by the simple rules of thumb animals and people actually use. These rules of thumb do not require a lot of information and do not perform complex tradeoffs; rather, they adaptively rely on human expertise, such as the knowledge of informative features and the use of core capacities (i.e., visual tracking or recognition memory). Surprisingly, in problems spanning economics, management, health and safety, simple rules of thumb can perform better than the more complex standard models. The future of decision theory depends on how wisely can researchers combine standard models and rules of thumb.