The quality of the municipal drinking water supply for the residents of Phnom Penh, Cambodia is assessed. Based on measurements of water quality for the source water, the treated water and the tap water taken over a 3-year period between 2009 and 2011, the evidence is that the water quality is good and meets both World Health Organization and national drinking water standards. This is despite only basic processes being employed in the water treatment process. This is possible because the source water is of good quality. However, developments in the Mekong river basin pose a number of threats to the quality of the source water. Source water quality degradation in terms of nutrients, heavy metals and pesticides is likely to impact the drinking water quality as the water treatment processes available do not address these species.