In the human body, there are three different structures referred to as tonsils. Most commonly, tonsils refer to the palatine tonsils; a pair of ellipsoid, almond sized structures located at the back of the throat. There are, however, another set of tonsils--the lingual tonsils--that are located under the tongue. In addition, there are adenoids (pharyngeal tonsils) that are embedded in the upper rear wall of the oral cavity. The adenoids or pharyngeal tonsils are often prominent in childhood but usually diminish in size in the adult. All of the tonsillar structures are part of the lymphatic system and contain lymphoid tissue.

As with other lymphoid tissue, the physiological function of tonsils is to process lymphatic fluid and to aid in the resistance to bacterial infection.

Palatine tonsils are embedded in mucosal membranous tissue of the mouth and throat, lie at the back of the mouth (oral cavity). Inflammation of these tonsils is commonly referred to as tonsillitis, a condition characterized by a painful swelling of the throat often associated with fever and difficulty in swallowing. Although minor tonsil infections are common, more severe infections can scar the tissue or produce an abscess (termed peritonsillar quinsy). In children and in the middle part of the twentieth century, tonsillectomy (the surgical removal of the tonsils) was a common minor surgical procedure for pre-pubescent children. Although still not an uncommon procedure, improvements in drug therapies and antibiotics, along with a realization of the important role tonsil tissue can play in the function of the immune system, has diminished the use of tonsillectomy in modern medicine.

Inflammation of the adenoids (pharyngeal tonsils) may also obstruct the nasopharynx region and lead to inflammation and infection of the Eustachian tubes or middle ear. When infections are severe or chronic, the adenoids are usually surgically removed (adenoidectomy).

The term tonsil can also refer to any small, generally oval-shaped mass of tissue. Accordingly, at the base of the cerebellum there is a rounded neural tissue mass termed the tonsil of the cerebellum. This neural tissues should not be confused, in either structure or physiological function, with the lymphatic tonsils located in the mouth.