Metallothioneins (MTs) are low molecular weight proteins, which are present in almost all types of organisms. In mammals, four main MT isoforms designated from MT-1 to MT-4 were identified. Their biological role, according to their characteristic structure, was shown to be mostly associated with cellular metabolism of metal ions, especially zinc. Moreover, the available evidence suggests broad regulatory properties of MTs in the control of cell senescence and various pathological processes including neurodegeneration, cardiovascular pathology, metabolic disorders, and various malignancies. This extensive review provides general in formation on the structure of MT family members and the cellular functions of MT-1, MT-2, and MT-4 isoforms as well as insights into divergent biological roles of MT-3. Due to the involvement of MT molecules in various processes related to carcinogenesis, an organ-specific presentation of current data concerning their potential impact on the progression of various tumors is given. The regulatory role of MT family members in the function of the immune system is also discussed in depth.