The Type II Cepheids include most intrinsic variables with periods between 1 and about 50 days, except for the classical Cepheids and the shortest semiregular variables of type M. The Type II Cepheids may be divided in groups by period, such that the stars with periods beween 1 and 5 days (BL Her class), 10-20 days (W Vir class), and greater than 20 days (RV Tau class) have differing evolutionary histories. The chemical composition of Type II Cepheids reflects the material they were made from as modified by their internal nuclear evolution and mixing. Finally, RV Tau stars are affected by mass loss by dust and species attached to the dust. The populations to which the various classes of Type II Cepheids are assigned constitute important clues to the origin and evolution of the halo of our Galaxy and the dwarf spheroidal systems from which at least part of the halo seems to have been accreted.