In the sciences, simplicity and unity are often held to be regulative ideals. In the following, I would like to make a case for a wider appreciation of the merits of complexity and pluralism. The reason is that a focus on simplicity and formal elegance may lead researchers astray as simple models may fail to faithfully grasp the diversity of nature and of human society. A complex model can sometimes provide a more fruitful framework. To illustrate this point, we will discuss examples from economics and from the neurosciences.It seems equally important to clarify what I do not intend. I do not wish to argue against reductionism. I do not see any good reason why biological or economic phenomena could not, in principle, be reduced to, say, processes in quantum electrodynamics. But, of course, all scientific disciplines have their specific perspectives, and that includes particular levels of explanation. Complexity may arise when, for the investigation of a particular phenomenon, complementary perspectives and various levels of explanation need to be used, and, particularly, when inter-level causal connections come into play.