Squamous cell carcinoma of the head and neck patients have a poor prognosis, despite progress in cancer therapy. The challenge faced by clinicians is to detect accurately patients with a high risk of relapse, hence who would need additional treatment and also to know if patients will respond to treatment. ctDNA has recently shown great potential to detect, monitor and increase comprehension in several cancer types. Results are however scare in squamous cell carcinoma of the head and neck. In this thesis, we lay out the advantages and challenges of using ctDNA in oncology and focus on the use in squamous cell carcinoma of the head and neck of ctDNA as a biomarker of minimal residual disease and of response to immune checkpoint inhibitors.