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 multidimensional 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.