This study produces a grounded theory of the process of communication with former partners for 30 men and women who share physical and legal custody of their children following divorce or separation. The formality of the custody arrangement was the core factor that influenced the ways in which study participants established and maintained boundaries regarding when, how, and what they communicated with their former partners. Other factors that played a role included the relationship with the former partner, the child, and the introduction of new partners. One cannot simply assume that because parents share physical and legal custody, they communicate with their former partners in a particular way; instead, the formality of the custody arrangement and the establishment of communication boundaries needs to be examined in order to understand how coparents communicate with their former partners.