Heritage speakers are individuals who were raised in a home where a language other than the dominant language of a given society was spoken. Heritage speakers are to some extent bilingual in both the language of their home and the dominant language. Only recently have linguists started examining heritage languages, languages that have not been completely acquired. In this article, I focus on features of heritage Polish spoken by "advanced high"¹ bilinguals from the Chicago area. A special focus is put on lexical and structural features that differ from standard Polish as described by grammars of Polish. I demonstrate that the Polish language undergoes reduction in nominal morphology and displays different patterns of verbal morphology. Structural innovations are modelled on English structure; however, new independent phenomena are also observed. Thus, the article contributes to the discussion about the role of incomplete acquisition, language attrition and language interference in shaping heritage languages.