Asphyxia or asphyxiation (from Greek α- "without" and σφύξις sphyxis, "heartbeat") is a condition of severely deficient supply of oxygen to the body that arises from being unable to breathe normally. An example of asphyxia is choking. Asphyxia causes generalized hypoxia, which primarily affects the tissues and organs. There are many circumstances that can induce asphyxia, all of which are characterized by an inability of an individual to acquire sufficient oxygen through breathing for an extended period of time. These circumstances can include but are not limited to: the constriction or obstruction of airways, such as from asthma, laryngospasm, or simple blockage from the presence of foreign materials; from being in environments where oxygen is not readily accessible: such as underwater, or in a vacuum; environments where sufficiently oxygenated air is present, but cannot be adequately breathed due to air contamination such as excessive smoke. Asphyxia can cause coma or death.

The body creates the need to breathe from the excess carbon dioxide in the lungs; the body has no way to detect the absence of oxygen. Many gases, though non-toxic, are classified as simple asphyxiants in their pure form or in high concentrations for this very reason.