Membrane technologies such as reverse osmosis (RO) have been used for over 20 years to treat seawater (SWRO) and waste waters. The applications, particularly for waste waters, have been limited due to the sensitivity of RO membranes to fouling and the ability of conventional pretreatment technologies to be effective. This paper looks at how pretreatment using microfiltration (MF) membrane technology cannot only reduce capital and operating costs on RO plants but also allow RO to be used on a wider variety of sources. This information is supported by operational data from a number of worldwide CMF-RO installations on different feed sources.