Thyroid aspiration is a method used to diagnose nodular thyroid disease. It has now been established as a highly accurate diagnostic method and is routinely used as the first step in the diagnostic process for nodular thyroid disease. Epidemiologic studies have shown that nodular thyroid disease is clinically common, with a prevalence of 4% to 7% of adults in North America and a new incidence of 0.1% per year, or approximately 275,000 new nodules per year in the United States. A recent survey of clinical members of the American Thyroid Association showed that the vast majority of them (96%) diagnosed thyroid nodules by fine needle aspiration cytology. Therefore, we estimate that 250,000 to 300,000 fine needle aspiration cytology examinations are performed each year in the United States alone. It is estimated that millions of fine needle aspiration cytology tests are performed worldwide each year. Therefore, the importance of fine-needle aspiration cytology in thyroid screening cannot be overemphasized. This chapter will describe biopsy techniques, cytologic diagnosis, complications, fine-needle aspiration results, diagnostic pitfalls, and other information useful in the clinical management of patients with thyroid nodules.