Oral malodor has been studied extensively in humans but very little work has been done in dogs where it constitutes a significant problem. In this article we review its causes, methods of detection, and strategies for preventing it. Oral malodor arises from microbial metabolism of exogenous and endogenous proteinaceous substrates in the oral cavity and is exacerbated by periodontal disease and poor oral hygiene. Gram negative bacteria found in plaque, in periodontal pockets, and on the dorsum of the tongue are primarily responsible for odor production. The volatile sulfur compounds (VSCs) hydrogen sulfide and methyl mercaptan, which are produced by these bacteria, are not only primarily responsible for the objectionable odor but have been implicated in the pathogenesis of periodontal disease. Assessment of malodor by portable sulfide monitors correlates well with organoleptic measurements. Reduction of microbial load in the oral cavity due to good oral hygiene practices (such as tooth-brushing) or by the use of appropriate diets or chews may reduce malodor formation.