The aim of this review is to consolidate various data about different functions of interleukin-11 (IL-11), a member of the IL-6 family. Numerous in vitro experiments have suggested a long list of IL-11 activities, including support and control of proliferation and differentiation of hematopoietic progenitors, as well as participation in osteoclastogenesis, neurogenesis and development of some other tissues. However, many of the in vitro effects of IL-11 have not been confirmed in experiments using animal models, hampering understanding of the physiological role of this cytokine. We discuss possible reasons for this apparent discrepancy between in vitro and in vivo data as well as perspectives of using conditional gene targeting to assess the role of IL-11 in ontogenesis and immune responses.