Skeletal muscle provides a fundamental basis for human function, enabling locomotion and respiration. Muscle loss occurs as a consequence of several chronic diseases (cachexia) and normal aging (sarcopenia). Although many negative regulators (atrogin-1, muscle ring finger-1, nuclear factor-kappaB (NF-kappa B), myostatin, etc.) have been proposed to enhance protein degradation during both sarcopenia and cachexia, the adaptation of these mediators markedly differs within both conditions. Sarcopenia and cachectic muscles have been demonstrated to be abundant in myostatin-linked molecules. The ubiquitin-proteasome system (UPS) is activated during rapid atrophy model (cancer cachexia), but few mediators of the UPS change during sarcopenia. NF-kappa B signaling is activated in cachectic, but not in sarcopenic, muscle. Recent studies have indicated the age-related defect of autophagy signaling in skeletal muscle, whereas autophagic activation occurs in cachectic muscle. This review provides recent research advances dealing with molecular mediators modulating muscle mass in both sarcopenia and cachexia.