Beside an indication of poor oral health, oral malodor has also tremendous effects to the patient’s mental status and social activities. More than 90% of the oral malodor is associated with intra-oral etiology. Oral cavities are habitats of hundreds of bacterial species. Mostly those who are able to produce volatile sulfur compounds are involving in oral malodor. A great majority of them are anaerobic bacteria found in periodontal pockets and tongue coating. This article reviews the causes of oral malodor associated with intra-oral bacterial communities, the methods which can be used to identify those species involved, and the future roles of probiotics in oral malodor treatment.