Many of our most troubling long-range problems---trade balances, substainability, AIDS, genetic defects, mental health, computer viruses---center on certain systems of extraordinary complexity. The systems that host these problems---economies, ecologies, immune systems, embryos, nervous systems, computer networks---appear to be as diverse as the problems. Despite appearances, however, the systems do share significant characteristics, so much so that we group them under a single classification at the Santa Fe Institute, calling them {\it complex adaptive systems (cas)}. This is more than terminology. It signals our intuition that there are general principles that govern all {\it cas} behavior, principles that point to ways of solving the attendant problems. Much of our work is aimed at turning this intuition into fact.
