Substitution of a party to the obligation - either a debtor or a creditor means that another person steps into the position of such party, but in a way that the obligation does not change its identity. In Serbian law, in legal relations intervivos, the creditor may be changed by means of (1) transfer of a contract as a whole; (2) assignment of a claim (cession); and (3) personal subrogation (payment with subrogation), which may be either statutory or based on a contract. The change of the debtor may happen within a wider scheme of (1) transfer of a contract as a whole; and as (2) substitution of a debtor, based on a contract. A contract for substitution of the debtor which is concluded by the creditor and a third party represents an external substitution (la reprise de dette externe). On the other side, if such legal consequence derives from a contract concluded by the debtor and a third party, it should be called an internal substitution (la reprise de dette interne). Substitution of the debtor which completely terminates the obligation of an initial debtor and gives rise to the obligation of the third party, represents a translative (or perfect, or privative) substitution. Conversely, substitution which means that a third party steps into the debt beside the initial debtor, should be recognized as a cumulative substitution. Serbian law of obligations contains special provisions relating to the internal translative substitution, as well as the provisions on external cumulative substitution of a debtor. There are no special statutory rules on cumulative substitution of a debtor on the basis of contract between the very debtor and the third party. However, this type of agreement may be concluded in accordance with the general principle of freedom of contract. The rules on stipulation for the benefit of a third party should apply here, mutatis mutandis. In addition, Serbian law does not contain any statutory provisions relating to translative external substitution of debtor. Still, the creditor and the third party may conclude such a contract freely, though its legal effects would be conditional upon the acceptance of the debtor, as under Serbian law the debtor may not be released from obligations without his consent.