The article deals with the issue of set-off and takes the form of comparison between the solutions adopted in Polish civil code and in Principles of European Contract Law. The article refers only to some of the legal matters connected with this institution's application. First of all its aim is to consider these features of set-off that significantly differ from solutions known in Polish law and other contemporary legal systems. Secondly, these differencies were treated as a pretence to a discussion on these problems connected with set-off that remain unexplained in Polish civil law doctrine. From this point of view four basic legal matters were considered: a) construction of set-off and its relation to prescription, b) set-off in case of assignment of claims, b) set-off of obligations expressed in different currencies, d) set-off in case of plurality of obligations and claims. At the end of each article's chapter the amendments to current civil code provisions were proposed.