The aim of this review is to collect various data on different functions of interleukin-11 (IL-11), a member of a family of IL-6-like cytokines. Numerous in vitro experiments have suggested a long list of IL-11 properties including support and control of proliferation and differentiation of hematopoietic progenitors, as well as participation in osteoclastogenensis, neurogenesis, and development of some other tissues. However, a great number of in vitro effects of IL-11 have not been confirmed in experiments using murine models, hampering an understanding of the physiological role of this cytokine. We discuss possible reasons for the divergence between in vitro and in vivo data as well as the perspectives of using conditional gene targeting to assess the role of IL-11 in ontogenesis and immune response.