In this paper, I address the problem of the enforceability of a claim for payment of a contractual penalty in the context of the admissibility of set-off of this claim. This problem manifests itself primarily in a situation where contractual provision regarding the penalty does not contain a deadline for its payment. The prevailing view in contemporary Polish case law says that in such a situation the obligation to pay the contractual penalty is indefinite, and therefore the claim for payment becomes due after the debtor is called to perform this obligation, which means that the creditor should first demand the payment of the contractual penalty. I tackle the issue from a historical perspective, starting from Roman law, through ius commune and doctrine views expressed on the basis of the Code of Obligations of 1933, to the modern doctrinal and jurisprudence concepts. These concepts, to the extent that they emphasize the need for prior demand for payment of a contractual penalty, establish a new requirement, breaking with the old tradition of understanding of the enforceability of a claim for payment of a contractual penalty.