Background: Although delineating the processes by which children appraise the family as a source of security from their collective experiences in the family subsystem has assumed center stage in many conceptualizations of child development, the dearth of measures of child adaptation in the family system has hindered empirical advances. Therefore, this study introduced and tested the psychometric properties of the Security in the Family System (SIFS) scales, a new measure designed to assess children's appraisals of security in their family as a whole. Methods: The SIFS was administered to 853 10-15-year-old schoolchildren and readministered to a smaller subsample two weeks later. Additional data was gathered from children, caregivers and teachers using a variety of instruments tapping family instability, cohesion, and conflict; parenting warmth and psychological control; child externalizing and internalizing symptoms; parent-child and interparental insecurity; and children's reactions to conflict simulations. Results: Consistent with models of emotional security in the family, exploratory and confirmatory factor analyses yielded three reliable (i.e., good internal consistency, test-retest reliability) dimensions of family security: Preoccupation, Security, and Disengagement. Concurrent and prospective associations between the SIFS scales and measures of family functioning, children's psychological problems, and insecurity in specific family relationships supported the validity of the SIFS. Support for the discriminant validity of the SIFS was evidenced by its specific patterns of relations with children's psychological problems and ability to predict psychological problems after controlling for insecurity in specific family subsystems. Conclusions: Results indicate that the SIFS is a psychometrically sound tool capable of advancing family process models, and that family security is a viable construct whose factors parallel already-identified patterns of children's security in other family relationships.