The most common type of blister under the tongue is a small mucous cyst. The sublingual mucosa has small glands that can form mucous cysts if they become blocked. Mucous cysts present clinically as “dome-shaped” intramucosal lesions, often on the lower lip, but also on the floor of the mouth under the tongue. Mucous cysts form as a result of disruption of the minor salivary gland ducts and leakage of mucin into the surrounding tissue, which is then surrounded by granulation tissue and cystified. Mucous cysts usually occur as a result of trauma. Mucous cysts have the potential to resolve on their own, especially in small or superficial areas. However, most mucous cysts require surgical removal. In the sublingual and floor of mouth areas there are also sublingual and submandibular glands. Inflammation of the glands or the presence of stones in the ducts of the glands can also cause sublingual puffiness. If any of these problems exist, they need to be seen in the Department of Maxillofacial Surgery at a dental hospital or the Department of Dentistry at a general hospital and usually require surgical treatment.