Family forms are changing and parental separation often constitutes the first in a series of marital transitions experienced by children. The first aim of the study was to address the nature of child adjustment in single-parent, reconstituted and intact families in middle childhood. The second aim was to examine possible group differences in children's adjustment several years after parental separation occurred during infancy or beyond. Another purpose was to explore whether child adjustment in different family types differed as a function of infant-mother attachment histories. The third aim was to identify concurrent and longitudinal factors linked to child adjustment. In particular, family type differences in children's sibling relationships and associations with adjustment were examined. Maternal parenting, expectations and children's appraisals of and attributions for maternal differential parenting in different family types were tested and related to sibling relationship quality. Associations between infant-mother and -sibling relationships with younger siblings' adjustment and sibling relationships in middle childhood were tested. The sample included 76 Caucasian families. In Study 1 mothers rated child adjustment using questionnaires and infant-mother attachment classifications were assessed by an observer using the Classic Strange Situation. A sub-sample of 55 mothers, target child and their biological sibling participated in Study 2 which involved maternal interviews about parenting and questionnaires about sibling relationships. Children completed questionnaires rating their sibling relationship and provided appraisals of and attributions for maternal differential parenting. Case studies of observed sibling interaction were conducted in middle childhood. Infant-sibling attachment and relationships were assessed using a modified Classic Strange situation and home observations respectively. Study I found that although children in single-parent families displayed higher levels of externalising behaviours than children in intact families, the effect size of the mean difference was modest and individual differences marked. Boys were less academically and socially competent than girls. Children who experienced parental separation from three years onwards displayed more externalising behaviours than children in intact families. Children with secure infant-mother attachment histories displayed lower levels of externalizing behaviours in all family types. One of the main hypothesis in Study 2 proposed that sibling relationships in families who experienced marital transition, especially single-parent families would be less warm and more argumentative. Contrary to prediction, levels of sibling warmth and conflict in single-parent families did not differ from those in intact families. Siblings in reconstituted families reported significantly less conflict than siblings in single-parent and intact families. Remarried mothers reported significantly higher levels of sibling warmth in their children' relationship than married mothers and less sibling conflict than singleparent mothers. Correlational analyses revealed that sibling warmth and conflict were related to some child adjustment dimensions, especially in reconstituted families. Univariate analyses revealed significant mean differences in sibling relationships as a function of maternal discipline strategies of sibling conflict, although variations according to family type and respondent were found. Although, mean levels of maternal differential parenting did not differ, varying associations, especially for differential control and sibling conflict occurred in different family types, suggesting a "process X context" relationship. Multivariate analyses revealed that maternal differential control accounted for 9% of the variance in siblings' externalizing behaviours. Mean differences in academic competence occurred according to siblings' offering the attributions "personal attributions and interests" and "don't know" as explanations for maternal differential parenting. Individual differences in the duration of infant solitary play infancy showed moderate stability with children's sibling relationships. Infant solitary and joint play was linked to younger siblings' externalising behaviours and competencies in middle childhood. Externalizing behaviours were associated with older siblings' positive and younger siblings' negative behaviours as infants.