The significant association with tobacco and alcohol combined with advanced age at time of diagnosis predispose head and neck squamous cell carcinoma (HNSCC) patients to increased risk of comorbidities. The presence of comorbidity affects treatment, treatment selection and subsequent outcome. Multiple studies have demonstrated comorbidity to be a strong prognostic factor for survival, and therefore comorbidity can be a major confounder in clinical trials. This review provides a summary of the current literature on comorbidity in head and neck cancer, measurements of comorbidity, the impact of comorbidity on treatment, treatment selection, and survival. A systematic search was performed in six electronic databases. In all, 31 papers were selected for this review. A meta-analysis on the prognostic impact of comorbidity was performed including 10 studies. Furthermore, 21 studies concerning comorbidity were reviewed. Several valid indices to classify comorbidity were described in the literature, none proven to be superior over the other. The prevalence of comorbidity increased with age and the presence of comorbidity influenced treatment and treatment selection. Furthermore, comorbidity was associated with lower socio economic status and increased the risk of early retirement after treatment. The meta-analysis on comorbidity as a prognostic factor, including 22,932 patients, showed that overall survival was significantly worsened among patients with comorbidity (HR=1.38 (1.32-1.43)). Increasing comorbidity-score was associated with increased risk of death. Comorbidity is important in HNSCC and significantly impacts on overall survival. Trials concerning HNSCC should always include information on comorbidity and randomized trials should stratify patients according to comorbidity in order to avoid bias in the study.