This study identifies key variables that distinguish nuclear families from step-families, and functional from dysfunctional stepfamilies. Sixty-three family triads (mother, father, child) were studied using five instruments: (1) Family Concept Test, (2) Locke-Wallace Marital Inventory, (3) Family Relations Test, (4) Family Interaction Task, and (5) background questionnaire. Results indicated that functional stepfamilies are similar to functional nuclear families in that both exhibit (1) good marital adjustment, (2) strong, positive bonds between biological parent and child, (3) disinclination to exclude family members, and (4) ability to make mutually compromised family decisions. The key differences were (1) less intense interpersonal involvement between the stepfather and child and (2) a stronger tendency toward the existence of parent-child coalitions in stepfamilies. Similarities between dysfunctional step-families and dysfunctional nuclear families include (1) stronger parent-child coalitions compared to their functional counterparts and (2) lack of mutual decision-making skills that fulfill the choices of individual members. Unexpectedly, marital adjustment was better in dysfunctional stepfamilies than in dysfunctional nuclear families. Relationship patterns were similar in functional stepfamilies and in dysfunctional stepfamilies except that they were more extreme in the dysfunctional stepfamilies. Results are discussed in terms of theoretical implications for understanding stepfamilies, and clinical implications in terms of how dysfunctional stepfamilies might best be treated.