The ability to differentiate adequately between true targets and false ones (discrimination) in the midcourse phase of attacking missiles flight is a key requirement for a successful ballistic missile defense. This ability is of particular importance for the proposed initial configuration of the Strategic Defense System (SDS phase 1) because (1) the preponderance of the reentry vehicle (RV) negations are to occur in the midcourse and (2) the effectiveness of the system is leveraged through selective (adaptive) defense that depends on reliable attack assessment in midcourse. Discrimination in SDS phase 1 is to be performed remotely by a combination of radar and passive optical sensors, and will be based on hoped for detectable differences in the emissions, shapes, and motions of the objects in a threat presumed to contain a variety of penetration aids accompanying the RVs. The BMD Panel of the Army Science Board has reviewed discrimination concepts, the current knowledge of threat characteristics and potential discriminants, the supporting experiments that have been conducted and are being planned, and the major uncertainties and open questions. This document includes observations, suggested action, and recommendations for the SDS phase 1.