Conduct problems in youth, including aggression, constitute a significant and increasing public health burden. Despite replicated evidence that conduct problems in girls predict numerous poor outcomes (e.g., health, substance use), research on aggression is focused almost exclusively on boys, thus perpetuating significant gaps in knowledge about risk factors, explanatory processes, and outcomes in girls. In boys, threat-biased social information processing is a correlate of aggressive behavior, particularly reactive aggression (i.e., retaliatory response to perceived threat/provocation). Elevated trait negative emotionality, the dispositional tendency to experience negative affective states such as anger and sadness, is a transdiagnostic risk factor for emotional and behavioral problems that may plausibly interfere with social cognition to promote aggression. Individual differences in autonomic functioning, including respiratory sinus arrhythmia (RSA), are also replicated risk factors for emotional and behavioral problems, as aberrant psychophysiological responding can have pronounced effects on emotional reactivity and regulation. Despite theoretical rationale and empirical support from research with boys and men, the associations of social information processing, elevated trait negative emotionality, and RSA with respect to aggression in girls remain largely unknown. To inform models of aggressive behavior in girls, the goals of these dissertation studies were to evaluate individual differences in negative emotionality, social information processing, and psychophysiology (i.e., RSA) as correlates of conduct problems, including separate dimensions of aggression (e.g., relational aggression). Using a sample of ethnically diverse pre-adolescent girls, Study I evaluated trait negative emotionality and threat-biased encoding and interpretation processes as sequential predictors of conduct problems. Preliminary evidence suggested that the hypothesized chain reaction effect marginally predicted both reactive relational aggression and general aggressive behavior. In the same sample, Study II assessed initial RSA and RSA withdrawal (i.e., reductions in RSA, also referred to as RSA reactivity), during a novel social information processing task, as correlates of conduct problems. Excessive RSA withdrawal was associated with higher levels of reactive relational aggression, and RSA initial values were inversely associated with rule-breaking behavior. Although these findings largely align with previous findings from male samples, reactive physical aggression was generally unrelated to aberrant social cognition and individual differences in RSA, unlike previous evidence based mostly on boys. Collectively, these preliminary findings fill critical gaps in knowledge on aggression and conduct problems in girls, informing the design of future studies that can propel targeted prevention and intervention approaches that are specific to risk factors and processes in girls.