The article aims at providing a synopsis of previous studies on the Kaunas Polish language, as well as identifying knowledge gaps, i.e., the things that – despite extensive research on this language variety – are still not known. At the turn of the 20th and the 21st century, intensified research both on the current Polish language variety, as spoken in the Kaunas region, and its history (mainly over the 19th century up till the 1950s) brought about vast amounts of linguistic and extralinguistic data. However, a detailed analysis shows that there is no easy answer to the question posed in the title of the paper. The article overviews the situation of the Kaunas Polish language, its territorial range, scope, and functions, as well as its defining features observable at the beginning of the 21st century. The author also outlines how the language variety – a subdivision of the Polish in the North-Eastern Borderlands – has evolved over the 20th century. Keywords: Polish language in the former North-Eastern Borderlands (Kresy) – language features typical of the North-Eastern Borderlands – substrate – linguistic borderland.