In the years ahead, social-behavioral SB modeling i.e., modeling that reflects behavior of individuals and social entities should help us 1 understand certain classes of SB phenomena with national significance 2 anticipate how those phenomena may plausibly unfold 3 estimate potential desirable and undesirable effects of additional events in the world or of possible U.S. or adversary interventions and 4 inform decision making. The phenomena of interest span a broad gamut that includes radicalization for terrorism, the weakening of democracy and national cohesion by foreign information operations campaigns, improving prospects for stability after international interventions, managing behaviors of populations after natural disasters, and dealing with opioid or obesity epidemics. Each such topic would be a good national challenge, as discussed later. Each has complex multi-dimensional social phenomena that are difficult to analyze without the unique power of modeling. In other domains, such modeling helps planners to strategize, plan, design, and adapt. It helps to avoid blunders and bad side effects of policy interventions. Todays SB modeling and related analysis is contributing far less to the study of such national issues than it could. Major advances are needed. But in what In this report we summarize the primary current shortcomings and obstaclessome inherent and some due to current methods and practices. We identified these obstacles through a review of recent trends and previous research in social-behavioral modeling and simulation, and through discussions and one-on-one conversations with leading experts in this area at RAND workshops and other conferences. In this report we then identify and discuss steps that deserve priority attention. Some of our suggestions build on earlier studies some are newer and more radical.