The article presents a review of current findings connected with phenomenon of cohabitation. It aims to define this form of close relationship between partners who do not decide to marry, but also between couples who treat cohabitation only as a temporary period. Nowadays, informal relationships are becoming definitely more and more popular. In the bigger urban areas, living together without a wedding is not a surprising solution. The article, based on the current results, tries to indicate how this solution affects the durability and overall functioning of a close relationship. The aim of the article is also to present current knowledge regarding differences in the level of durability and satisfaction with formal and informal relationships.