Any effort to change a complex social system should begin by constructing models and simulations of the problem and its social context. These will increase the likelihood of long-term success, decrease the chance of unexpected negative side effects, and help conserve scarce national resources. Models may be of three types: (1) prose models--which are descriptions and theory-statements that attempt to explain the structure and dynamics of the factors affecting the problem within its setting; (2) structural models--which use a variety of symbol systems to represent the elements and relationships affecting the problem; (3) dynamic models--which are built upon the other two but have the characteristic that they can demonstrate problem-critical behaviors of the system repeatedly over an extended time period. Modeling and simulation techniques have strengths and weaknesses, and techniques may be combined to provide the best insights and approaches to practical social problems. (Author/DAG)