This paper reviews the physical metallurgy and damping characteristics of copper-manganese (Cu-Mn) alloys. Many of the examples given in this paper are based upon the authors' experimental investigations of an alloy with nominal composition (composition in weight percent) of 48Cu-48Mn-1.5Al-0.27 Si-0.072Sn-0.028C-0.05Er. The results for this material were obtained in a bending mode, and the reported strain amplitudes represent the maximum surface strain imposed. Damping in these materials is very sensitive to heat treatment and carbon content. Recent work has shown that carbon in solid solution is detrimental to the damping capacity and results in a strain aging behavior and subsequent loss of damping capacity when stored at room temperature. Copper-manganese alloys can lose more than 50% of their damping capacity during room temperature storage. The role of erbium in the stabilization of the damping capacity is explained in the context of the physical metallurgy of the Cu-Mn system.