This paper investigates algebraic homological localizations which have arisen in the homotopy theoretic study of localizations of spaces. It focuses on the "HR-localization" of groups and the "HZ localization" of modules over a group [capital Greek]Pi, where the coefficient ring [italic]R is allowed to be a subring of the rationals or a finite cyclic ring. The investigation is based on a construction of natural transfinite towers which eventually stabilize to the desired homological localizations. These towers are used to show that the HR-local groups form the smallest class of groups containing the trivial group, closed under inverse limits, and closed under central [italic]R-module extensions.