Collective intelligence allows groups of individuals to solve problems which otherwise could not be solved by a single individual. Insect workers have tiny brains, but by functioning as part of a self-organized colony, they find sophisticated solutions to vital organizational problems (e.g., finding a suitable new home or exploiting the best food sources in a changing environment). In consensus decision making, unanimity among workers is crucial. In contrast, combined decision making requires that different groups of workers within the colony chose different options. Communication and learning are often fundamental in collective decision making. However, as workers gain experience, communication may lose importance as an information source for workers. How social insects collectively solve problems parallels decision making in other biological sys- tems (e.g., neuronal networks), and investigation into social insect collective decision making has inspired new solutions to optimization problems in areas such as computer sciences and the organization of communication networks.