Technical advances in high-resolution ultrasound have brought ultrasound to the forefront of thyroid imaging. Despite the seeming similarity in the imaging appearance of diffuse thyroid diseases, the radiologist should be able to identify patients with diffuse thyroid abnormality, be aware of the spectrum of the imaging appearance of common thyroid conditions, suggest a clinically relevant differential diagnosis, and exclude nodular disease when present. This article presents an overview of recent developments in ultrasound and discusses techniques for performing a neck ultrasound. The clinical and imaging features of the most common diffuse diseases affecting the thyroid gland are reviewed.