Rainwater harvesting is the primary source of fresh water on the majority of isolated and volcanic islands in the Eastern Caribbean island chain that lack reliable sources fresh water. Recent research studies have determined that the microbiological quality of rainwater in cistern systems is generally unstable and can lead to serious health implications when used for human consumption. Another related issue is that rainwater shortages occur more frequently due to fluctuating rainfall patterns and increasing rainwater consumption rates. In this paper minimum design requirements are determined, using data from the islands of Saba and St Eustatius, for the construction of new domestic rainwater systems on small volcanic islands in the Eastern Caribbean. These minimum design requirements can be used to prevent potential water quality and quantity issues. One of the conclusions of this research study is that mainly the collection and storage elements of rainwater systems need to be constructed using non-toxic and non-corrosive materials to mitigate potential health risks. Also, first-flush devices need to be installed in the collection system to divert the mostly contaminated first load of rainwater after a dry period. The storage tanks need to be watertight and protected from sunlight, preferably located at least 30 meters away from active bacterial sources (such as open street sewers or cesspits). The minimum rainwater catchment area for a standard rainwater system, based on a 4-person family with a modern water consumption pattern, is 113m2; the minimum cistern storage capacity is 24m3.