Aim: To systematically review the different causes of halitosis, both intra oral and extra oral. Methods: PubMed-MEDLINE was searched to identify potential relevant studies. The keywords used to search were "halitosis", "bad-breath" and "oral malodour causes". The inclusion criteria involved articles published between 2009 and 2014, written in or translated to English which included reviews, cross-sectional studies, randomised clinical trials, non randomised trials, prospective observational studies and case-control studies. Results: From the 465 studies initially selected, when the inclusion criteria was applied and reviewed by the authors, 39 articles provided relevant information about the aetiology of halitosis. This included 11 reviews, 12 cross-sectional studies, 7 randomised and 2 non randomised trials, 2 prospective observational studies and 5 case-control studies. Conclusion: Halitosis has a complex aetiology, although majority of the studies agree that in 90% of cases, it is linked to an intra oral cause, specifically when Volatile Sulphur Compounds (VSC) are produced by bacterial decomposition. Coating of the tongue and periodontal disease are the main aetiological factors. More research has to be done to clarify the exact pathophysiological mechanism of halitosis.