IL-4 is a pleiotropic cytokine which plays a pivotal role in shaping immune responses. The effects of IL-4 are mediated after binding to high affinity receptor complexes present on hematopoietic as well as non-hematopoietic cells. This review will summarize the current knowledge on the molecular structure of the different types of IL-4 receptor (IL-4R) complexes as well as the signal transduction mechanisms induced by IL-4 leading to cellular proliferation and / or gene activation. IL-4 effects are modulated by soluble forms of the respective receptor molecules which are produced by several immune cells in a regulated manner. The biological impact of recently described IL-4R allotypes of mice and humans as well as the results of studies with IL-4R knockout mice will be particularly emphasized.