Guided by the emotional security hypothesis, this study reports on the development of a new self-report measure that assesses children's strategies for preserving emotional security in the context of interparental conflict. Participants were 924 sixth, seventh, and eighth graders and a subset of their mothers, fathers, and teachers. Exploratory and confirmatory factor analyses of the Security in the Interparental Subsystem (SIS) Scale supported a seven-factor solution, corresponding well to the three component processes (i.e., emotional reactivity, regulation of exposure to parent affect, and internal representations) outlined in the emotional security hypothesis. The SIS subscales demonstrated satisfactory internal consistency and test-retest reliability. Support for the validity of the SIS Scale is evidenced by its significant links with parent reports of children's overt reactivity to conflict, children's responses to interparental conflict simulations 6 months later, and children's psychological maladjustment and experiential histories with interparental conflict across multiple informants (i.e., child, mother, father, and teacher). Results are discussed in the context of developing recommendations for use of the SIS and advancing the emotional security hypothesis.