The article shows a progress in researches on Polish parliamentarism in the period of the Polish – Lithuanian Commonwealth. The literature on the old Polish Sejm is currently very extensive. On the basis of archival printed and manuscript sources (especially Sejm’s diaries), the authors prepared monographs of individual parliaments, then the synthesis of longer periods in the history of the parliament, and finally they became interested in particular problems of the parliamentary system. The main examples of the research directions are indicated. In the conclusion, the author suggested, that it would be appropriate to examine specific issues such as the nature of the parliamentary mandate, because to this day, views from the 19th century are reiterated in historical literature.