This review deals with the different forms of halitosis. Halitosis can be subdivided according to its original location. At present, halitosis of oral origin is quite well understood and some excellent reviews have already appeared in the literature. Special attention is given here to extra-oral halitosis. Extra-oral halitosis can be subdivided into: halitosis from the upper respiratory tract including the nose; halitosis from the lower respiratory tract; blood-borne halitosis. In blood-borne halitosis, malodourant compounds in the bloodstream are carried to the lungs where they volatilise and enter the breath. Potential sources of blood-borne halitosis are some systemic diseases, metabolic disorders, medication and certain foods. The methods of analysis of halitosis are critically reviewed. Attention is also given to odour characterisation of various odourants.