A French kiss, tongue kiss or deep kiss is a romantic or sexual kiss in which one participant's tongue touches the other's tongue (or lips) and usually enters his/her mouth.

Although family members may sometimes kiss on the lips, a kiss using the tongue almost always indicates a romantic or sexual relationship. French kissing stimulates the lips, tongue and mouth, which are all areas very sensitive to touch. It is considered by many to be both very pleasurable and highly intimate. Unlike other forms of "casual" kissing (such as brief kisses of greeting or friendship), episodes of French kissing will often be prolonged, intense, and passionate. It may signal the beginning of a period of "making out" or sex. Because of the intimacy associated with it, in many regions of the world tongue kissing in public is not acceptable to most, particularly for an extended time.

In a tongue kiss participants may exchange saliva, which would often be considered disgusting in other contexts, but which may add to the passion and excitement of the sexual kiss. Although most sexually transmitted diseases are not transmitted by kissing, the exchange of saliva in a French kiss may increase the chances of catching an orally transmitted disease.[citation needed] Infectious Mononucleosis (American: Mononucleosis or, colloquially, "mono"; European: Glandular Fever), a disease spread through saliva, is often colloquially referred to as "the kissing disease."

French kissing is commonly used in movies to show willing sex between lovers.[citation needed] It is often used by lovers to express their intimate feeling toward each other and often precedes sex (and frequently continues during sex).[citation needed]

French kissing has been traditionally associated with Western societies, but is now becoming quite popular around the world.[citation needed]

According to contemporary Hindu religious beliefs, a French kiss can break a "Fast" since it involves exchange of saliva.