ABSTRACT
This chapter, which is meant as an introductory tutorial and brief literature survey, has four main sections. The first describes complex systems and lists their defining characteristics, and motivates the interest in validating models of complex systems by discussing validation risk. Then, each of the following sections discusses one of three selected defining characteristics of complex systems (sensitivity to initial conditions, emergent behavior, and composition of components), explaining why the characteristic in question makes modeling and validation more difficult and offering some approaches to dealing with and mitigating the difficulties.
