The author analyses the legal treatment of the penalty clause into Spanish Civil Law, with special attention to the regulation offered by the Civil Code in their articles 1152 to 1155, the differents functions which the penalty clause achieves and the differentiated regulation into Fuero Nuevo of Navarra in the Lex 518. All of this can be seen as a reflection of the dualism in Spanish Legal System between the Common Civil Law (represented by the Spanish Civil Code) and the Foral or Special Law (represented by differents regionals compilations). The regulation of the Spanish civil code sets as general rule the contractual penalty as substitute of the regime of compensation into the article 1101 of spanish Civil Code in case of breach of contract or in case of defective or untimely performance by the debtor. This provision allows to the creditor to avoid a real damage assessment, so that the penalty clause lets a advanced estimate of damages without needing to prove them. But also admits the regulation of the penalty clause into the spanish Civil Code other functions, for instance the cumulative penalty (with the legal and previously valued compensation of damages or with the specific performance). This regulation, unchanged since the enactment of the spanish Civil Code, has been object of special attention by the preliminary draft to modernisation of the law of obligations drawn up by the Commission of Codification of the Ministry of Justice in 2009, picking up some of the guidelines present in other European Civil Codes as well as in the contributions of prestigiouses scholars, which are expression of the effort aimed at the harmonisation of European Contract Law.