Halitosis is a term that refers to an unpleasant or foul odour originating from the oral cavity that can be caused by either intra-oral or extra-oral factors. Despite the fact that halitosis has multifactorial aetiology, intra-oral factors play a significant role in the majority of cases. This systematic review assesses halitosis's intra-oral and extra-oral associations. An electronic search through MEDLINE (PubMed), Google Scholar and the Wiley Online Library was conducted to identify relevant manuscripts. A keywords-based search was performed, using the terms ‘halitosis’, ‘bad-breath’, and ‘oral malodour causes and aetiology’. Articles published from January 2014 to December 2020 were included. We selected studies evaluating the intra-oral and extra-oral factors that induce oral malodour, as well as the factors associated with systemic diseases. Eighty to ninety percent of halitosis is caused by intra-oral factors, with coated tongue, periodontal diseases and poor oral hygiene practices being the principal factors. Ten to twenty percent of halitosis is induced by extra-oral factors associated with systemic diseases. Multiple factors can cause halitosis, but most of the aetiology is intra-oral. Increased medical awareness is needed to determine the actual pathophysiological process of oral malodour in otherwise healthy individuals.