Reverse osmosis (RO), sometimes known as hyperfiltration, is an efficient method of removing salts, chemical contaminants (such as nitrate, fluoride and carbonates), micro-organisms and heavy metals (primarily lead and copper), from tap water creating essentially, pure water. The principal employed is the reverse of natural osmosis.

Water is forced through a semi-permeable membrane by a high-pressure pump. Purified water permeates the membrane and is collected while concentrated water, containing dissolved and undissolved impurities, is discharged to the drain. The process allows the removal of more than 95% of "contaminants" and particles as small as ions from the water.

Most reverse osmosis systems use a process known as crossflow to allow the semi-permeable membrane to continually clean itself. Not all water passes through the membrane. Some is required to flow over the membrane sweeping away rejected impurities. Therefore, from one stream of water, an RO machine produces one purified stream called "permeate" and a second stream called "concentrate", "brine", or "reject". The percentage pure water collected depends on such things as the membrane polymer type, membrane condition (e.g. age, cleanliness), and driving pressure, but is normally only around 5% of the source water content.

Although bacteria are too large to pass through RO membranes, RO technology should not be used for bacteria control. Bacteria can grow through a membrane over time. To counter this effect, the membrane should be replaced periodically alternatively, an in-line Ultra Violet (UV) chamber can be added to most RO systems. Additionally, a carbon post filter can sometimes be attached to remove undesirable odours or colouration. This is a very popular combination of filtration technologies to achieve the highest levels of purification.

Reverse osmosis water is one of the purest forms of water that can be gained without the use of chemicals or heat. It is also a much cheaper to produce than bottled water, yet has a similar or a higher level of quality. However, the initial cost of the RO unit, the amount of concentrate produced and the time taken to collect RO water often makes these systems unattractive to the hobbyist (systems typically only produce 20-30 gallons of pure water per day).