This article describes a standardized protocol for repeated social defeat stress in mice, which is a useful model for studying affective disorders. The protocol involves C57BL/6J mice being subjected to bouts of social defeat by a larger and aggressive CD-1 mouse, leading to the development of a depressive-like syndrome characterized by enduring deficits in social interactions. The protocol consists of three stages: selecting aggressive CD-1 mice, conducting agonistic social confrontations, and confirming social avoidance in subordinate C57BL/6J mice. The automated detection of social avoidance increases throughput, reproducibility, and quantitative analysis. The protocol is adaptable but typically requires 3-4 weeks to complete. The model has excellent etiological, predictive, discriminative, and face validity, making it a valuable tool for understanding the molecular mechanisms underlying affective disorders.This article describes a standardized protocol for repeated social defeat stress in mice, which is a useful model for studying affective disorders. The protocol involves C57BL/6J mice being subjected to bouts of social defeat by a larger and aggressive CD-1 mouse, leading to the development of a depressive-like syndrome characterized by enduring deficits in social interactions. The protocol consists of three stages: selecting aggressive CD-1 mice, conducting agonistic social confrontations, and confirming social avoidance in subordinate C57BL/6J mice. The automated detection of social avoidance increases throughput, reproducibility, and quantitative analysis. The protocol is adaptable but typically requires 3-4 weeks to complete. The model has excellent etiological, predictive, discriminative, and face validity, making it a valuable tool for understanding the molecular mechanisms underlying affective disorders.