What causes tongue swelling?

A few possibilities: Tongue swelling, either part or all of the tongue, can be due to many causes, such as: allergic reactions (may be a 911 emergency), irritants (chewing on leaves containing crystals), infections (from piercings), trauma (bit tongue), enlarged lingual tonsils (tongues have tonsil-type tissue in the back portion), diseases that deposit substances inside the tongue, tumors, burns, etc...

Many things: Dr. Kwok's answer is thorough. Keep in mind tongue swelling *can* be a life threatening problem if severe. If you have severe swelling, feel like you are having trouble breathing, call 911 or head to the er. If you take an ace-inhibitor (lisinopril or other -pril medicine) make sure you get this checked out asap as those medicines can cause a very rare but serious allergic reaction.