Posttreatment imaging is done when a recurrent tumour is suspected, to confirm the presence of such a lesion and to determine its extent. The extent of a recurrent cancer is important information for determining the possibility of salvage therapy. Imaging may also be used to monitor tumour response and to try to detect recurrent or persistent disease before it becomes clinically evident, possibly with a better chance for successful salvage. This article reviews the expected imaging findings after treatment of head and neck squamous cell cancer, and how to differentiate these from persistent or recurrent cancer. The relative value of anatomical and biological imaging modalities, including newer techniques such as diffusion-weighted magnetic resonance imaging, is addressed. The imaging findings in treatment-induced complications, such as tissue necrosis, sometimes difficult to differentiate from cancer, are explained.