The main purpose of this paper is to investigate the influence of maternal and paternal languages on the development of a bilingual child’s language. It is assumed that a mother’s language has the greatest influence on a child’s language. In addition, maternal language is responsible for the largest part of the variety in the language children understand and use as well as the development of bilingualism. We also believe that fathers influence the language of a home and mothers influence the language of a child. In our analysis, the focus is on the quantity and quality of exposure to the given languages, the pattern of language used by parents, the language of formal instruction and the style of parentchild interaction. Over one hundred and fifty case studies were under examination depicting various language constellations, such as e.g. Polish and English, Polish and Russian, Polish and German, Polish and Ukrainian, Polish and Belarusian, Polish and Italian. Hence, with this paper we hope to be able to better understand cross-linguistic influence on bilingual speakers.