Many of our most pressing challenges, like managing ecosystems and economies, or preventing mass epidemics and market crashes – require a better understanding of complex systems. In recent years, the science of “complexity” came into existence and has grown rapidly. “Complexity” has a precise meaning in science. We call a system “complex” if the whole transcends the parts and if multiple agents participate in it. Most complex systems consist of diverse entities that interact in space and in time; they can be real or virtual. Examples of complex systems are ecosystems, cities, universities, or the stock market. Systems that are merely based on a feedback loop, like automatic temperature regulators, are not complex. Complicated systems are also not necessarily complex. Complicated systems may have diverse parts or many variables, but they are not adaptive.