This summary of theoretical, scientific, and professional writings is focused on three questions related to determining the best interests of the child whose custody is disputed following a separation: (1) What are the stakes when it comes down to the best interests of the child? (2) What are the characteristics of the situations that end up being analyzed or debated in the courtroom? (3) How is this principle assessed from a practical point of view; what weight is given to the various criteria examined by the judges and the specialists? As concerns the professionals whose task it is to come to a decision as to the best interests of the child, the answers to these questions highlight a range of challenges that underlie their being regulated by a flexible yardstick that provides few clear guidelines within a context itself often extremely controversial and complex. An examination of these writings concerning the practices of judges and specialists reveals two conflicting currents of thought: the importance of the referral parent, more especially when the children are very young, and the trend towards co-parenting and an equal sharing of custody between the parents. It is on the basis of these conclusions that suggestions for future research are made.