Infrared astronomy has come into its own over the last decade. Based on mature detector technology and sophisticated instrumentation it is contributing exciting science in many fields of astrophysics. Stellar evolution is a field that has long been dominated by ultraviolet and optical work, but one that has benefited from a strongly increasing contribution from the infrared (IR) and sub-millimeter (sub-mm) domains. In particular, spectroscopy in these domains holds the promise to enable important advances through quantitative analysis of individual stars and stellar systems.