There has been a growing interest, especially in developing countries, in rooftop rainwater harvesting as an alternative source of drinking water. This paper reviews the available information on the water quality of rooftop rainwater harvesting systems. Various factors that affect the physico-chemical and microbiological quality of harvested water are discussed. Different contaminants including heavy metals and trace organic pollutants in the roof runoff reported from different parts of the world are compared. The review shows that the quality of harvested water from roof catchments often does not meet the drinking-water guideline values. Most of the studies reveal that harvested water is heavily contaminated microbiologically by a variety of indicator and pathogenic organisms unless special care is taken during collection and storage of rainwater. Heavy metals and trace organics could also pose problems in some cases. The review thus indicates that the purity of rainwater harvested from rooftops should not be taken for granted, and analysis of the harvested water especially for microbiological contamination should be undertaken. Appropriate treatment of collected rainwater would be necessary to make the harvested rainwater fit for drinking. The review also shows the need for further research on proper design and maintenance strategies to minimise contamination of roof-collected rainwater supplies.