Computed tomography (CT)-defined sarcopenia, or low muscle mass, is a demonstrated poor prognostic factor for survival in patients with cancer, however, its impact in patients with head and neck cancer (HNC) has only recently been established. Sarcopenia may be considered a nutrition-related condition that can occur in either a primary, age-related context or secondary to inactivity, malnutrition or a range of complex pathogeneses including malignancy. This presentation will provide an overview of CT-defined sarcopenia and the latest research on the prognostic impact of CT-defined sarcopenia on outcomes for patients with HNC with a specific focus on survival in those undergoing radiotherapy ± other treatment modality of curative intent. This presentation will demonstrate that: i) CT-defined sarcopenia impacts negatively on overall survival in patients with HNC and holds a clinically meaningful prognostic value; ii) consensus regarding sarcopenia assessment and skeletal muscle index threshold values is warranted; and iii) future research should focus on implementation of clinically relevant prognostic muscle mass evaluation into routine care.