Work-to-family conflict can result from incompatible demands and resources of work and family roles, so that participation in the family role is more difficult because of participation in the work role (Grandey & Cropanzano, 1998; Voydanoff, 2005). Previous theorizing suggests that the interplay between resources and demands is of particular importance: Congruence of resources associated with one role (e.g. the family role) to meet demands of another role (e.g. the work role) can make participation in both roles effective (DeBord, Canu, & Kerpelman, 2000; Voydanoff, 2005). Thus, resources and demands do not operate independently from each other. Few studies investigated determinants of work-to-family conflict in terms of an interplay between demands and resources. Furthermore, most research in this domain is cross-sectional, so it is difficult to draw conclusions about causality. Therefore we studied the interactive influence of resources and demands on work-to-family conflict in a longitudinal study spanning eight years. Further, we analyzed couples because previous research and theorizing suggests that work-to-family conflict is the result of a dynamic process that unfolds within the couple. In the present study, we used data from the Swiss Household Panel (SHP), i.e., a yearly panel study which follows a random sample of households in Switzerland. More specifically, we analyzed perceptions of work-to-family conflict and indicators of changes in role demands and resources in a subsample of 705 couples, over eight measurement points. Results illustrate how interactions between resources and demands (e.g. between partner support and long working hours; or between autonomy at work and household demands) affect perceived conflict within couples. These analyses provide important insights about the causes of work-to-family conflict. Also, they have important implications for designing effective organizational policies intended to improve work-family balance or to help couples develop adaptive strategies to improve the balance between family and work.