DynNetSim is a dynamic network software platform for modeling and analyzing social and other networks as they evolve over time. DynNetSim combines stochastic modeling with a behavioral simulation framework that synthesizes system dynamics and computational agent paradigms. The resulting framework provides a holistic depiction of networks responding to influences of environmental forces, trends, and disruptive events. In social networks comprised of intentional (goal-directed) entities such as terrorist groups, such responses may encompass opportunistic and adaptive behaviors. DynNetSim also enables exploration of the likely impacts of prospective strategies to change networks, such as attacking adversaries' social networks or reducing vulnerabilities in our own critical infrastructure systems. DynNetSim can enhance diverse counter-terrorism activities including intelligence analysis; critical infrastructure protection; and preparedness planning.