This article concerns the long-term cooperation of Simon de Terano, a consistory lawyer, with Polish diplomats at the Holy See and the councils of Constance and Basel. While studying law in Padua under the supervision of Franciszek Zabarelli and crowned in 1411 with a doctorate in canon and Roman law, Simon de Teramo met outstanding Polish clergy (including Andrzej Łaskarzyc, Paweł Włodkowic and Piotr Wolfram). Before 1414, he was appointed a consistory lawyer at the Roman Curia. Following the recommendation of the Archbishop of Gniezno, Mikołaj Trąba, Polish King Władysław Jagiełło appointed Szymon as a lawyer at the Council of Constance in 1414. At the council, Szymon actively cooperated with the Polish delegation, trying to solve several problems related to Poland's conflicts with the Teutonic Knights. At that time, Poland sought an official condemnation of Jan Falkenbeig's satire, which perversely called for the murder of King Jagiełło and the extermination of his people. After the council, in the years 1419-1422, Simon defended the Polish cause in the Roman Curia during the trials of the Teutonic Knights opposing Poland, which took place before Sigismund of Luxembourg, as well as at the Holy See. Gdańsk Pomerania, the areas of Chełm, Michałów and Żmudź were the subject of a Polish-Teutonic dispute. Simon of Teramo was present for the first time as a Polish lawyer at the Council of Basel in 1433-1435.