Children's relationships with their parents are considered to be a major influence in their development. Parents' use of discipline with their children, in particular, constitutes a major component of the parent-child relationship and is linked to children's later social skills and behavioral and emotional functioning. Prior studies have shown that parents treat their children differently. The purpose of this dissertation was to examine whether parents use differing levels of discipline with their children and to delineate parent, child, and contextual factors associated with differential parental discipline. This study utilized a behavior genetics twin study design to control for extraneous influences, such age and life changes, which may significantly impact a parents' use of discipline. Participants in this study consisted of 38 families (36 twin pairs, 2 sets of triplets) who had previously participated in the Southern Illinois Twins and Siblings Study (SITSS; DiLalla, 2002) and were now between the ages of 5 ½ to 10 ½. Children and their parents were each asked to report on various parenting practices and discipline techniques used within their home. Parents completed a variety of questionnaires assessing socioeconomic status, interparental conflict, parent personality, child temperament, and child emotional and behavioral functioning and mothers tracked their discipline for one week through a diary. Results from this study showed that children living in the same family experience different levels and types of discipline. Child temperament and emotional/behavioral functioning were related to differential discipline practices. The twin in the family rated as exhibiting greater levels of extraversion, effortful control, and prosocial skills received more sensitive parental discipline. In contrast, the twin showing more conduct problems, peer problems, emotional symptoms, and inattention/ hyperactivity experienced harsher parental discipline. Importantly, parents and their children did not agree on the amount of differential discipline used in their homes. Parents reported using similar levels of discipline with both of their children and children reported large differences in the type and amount of discipline shown to them and their co-twins. Findings from this study could add to the effectiveness of many parent training programs by helping parents to understand the interactions between various parent, child, and contextual characteristics in the initiation of parental disciplinary strategies. Through this understanding, parents may begin to match their disciplinary styles to best meet their children's phenotypic characteristics and needs and environmental demands. Lastly, the search for various genetic and environmental factors associated with harsh discipline practices will greatly enhance the success of prevention programs. By being able to determine familial and contextual characteristics associated with harsh or aversive punishment practices, we may be able to predict those parents most at risk for harsh discipline and to intervene before abusive discipline practices are used.