Participants in legal transactions with the aim of securing the contract or the contractual discipline of the parties often contract the institute of contractual penalty. By the term contractual penalty, we mean a certain amount of money or other material benefit that the debtor undertakes to pay or obtain to the creditor in case of non-performance, delay or irregular performance of the obligation. A contractual penalty can be contracted as an agreement on a contractual penalty or as a clause within the contract of the main obligation whose fulfillment it insures. One of the most important characteristics of a contractual penalty is its accessory nature, or else called secondary nature, because the contractual penalty agreement follows the legal fate of the main obligation. Although the contracting parties are in principle free to determine the amount of the contractual penalty at their will, the court may, at the debtor's request, reduce the amount of the contractual penalty if it finds that the amount is disproportionately high considering the value and meaning of the object of the obligation. The realization of the creditor's rights under the contractual penalty depends on the case for which it was contracted. Therefore, if the contractual penalty is agreed upon in case of non-performance of the obligation, the creditor can demand either the fulfillment of the obligation or a contractual penalty. On the other hand, when the contractual penalty is contracted for the case of delay or irregular performance, the creditor has the right to demand both the fulfillment of the obligation and the contractual penalty. In any case, the creditor will exercise his rights in a simple and easy way, because he is not obliged to prove the existence or the amount of the suffered damage, nor the guilt of the debtor, given that it is presumed. The basic purpose of this institute - protection of the creditor's interests arises from that. Apart from the characteristics of the contractual penalty, the paper also presents the relationship between the contractual penalty and related claueses.