We discuss the band structure and the optical, electrical, and magnetic properties of the compound semiconductor Hg1−xMnxTe, formed by substitution of the magnetic Mn ions for Hg in the HgTe lattice. In the absence of a magnetic field, this alloy is very similar to its sister compound Hg1−xCdxTe. Although Mn is not a group II element, Hg1−xMnxTe crystallizes in the zincblende structure, forming good quality crystals up to x≊0.35. Its energy gap and related band parameters vary with x, but at a rate about twice as fast as in Hg1−xCdxTe. The electrical properties of Hg1−xMnxTe are again similar to those of Hg1−xCdxTe, exhibiting, e.g., electron mobilities in excess of 106cm2/V⋅s. Because Mn is a magnetic ion, Hg1−xMnxTe differs from Hg1−xCdxTe in its magnetic properties, as well as in the behavior of its electrical and optical properties in the presence of a magnetic field. For example, for x≳0.17 Hg1−xMnxTe exhibits a transition to the spin glass phase at low temperatures. Furthermore, the presence of Mn ions leads to an exchange interaction between the localized magnetic moments and the band electrons, which in turn affects the band parameters and leads to new and spectacular effects in the transport and optical properties.