25% of the population have bad breath. This can be either Foetor ex ore (90%) or Halitosis (10%). Foetor is only perceptible in mouth breath and its causes lie in the oral cavity. Halitosis is detectable in oral and/or nasal breath and stems from either the nose/pharynx (local Halitosis; nasal breath only), the lung or, rarely, the stomach (systemic Halitosis; mouth- and nose breath). A respective differential diagnosis is prerequisite for causal therapy. Foetor ex ore is caused by volatile sulphur compounds produced by proteolytic microorganisms in the oral cavity, especially on the dorsum of the tongue. Prophylaxis and therapy consequently build on mechanical and chemical reduction of these bacteria. Bad breath may become a serious social handicap and shall not be turned into a taboo.