The article analyses the extent to which the UNIDROIT Principles of International Commercial Contracts (UPICC) are used to interpret and supplement Czech contract law. Under Czech legal doctrine the UPICC are part of lex mercatoria and not considered as a generally binding set of legal rules. However, contracting parties are free to make them part of their contract. The authors carry out a comparative analysis of selected UPICC rules and their counterparts in the Czech national law (Czech Civil Code) relating to negotiations in bad fairh, surprising terms, currency of payment, right to terminate the contract and interest for failure to pay money., Monika Pauknerová, Magdalena Pfeiffer., and Obsahuje bibliografické odkazy