Large advancements in Reverse Osmosis (RO) membrane formulation, casting and assembly techniques have resulted from the high demand of this system since the 70s. Such improvements have led to more durable and efficient membrane, however it is not possible to produce high-purity water with a single-pass through an RO membrane due to the subsequent treatment required involving a full train or mixed bed Ion Exchange (IX). This article reviews the fundamentals of double-pass RO and includes a comparison to single-pass RO.