In the following article Anthony Bennett, freelance writer for Filtration+Separation, defines high purity water, reviews technologies available for Its production and describes four case studies from the USA and the UK where various process combinations have been used to generate high purity water of various grades. He also demonstrates how reverse osmosis (RO) is increasingly being used to deionize raw water for feeding on to either electro-deionization or mixed bed ion exchange 9 technologies for further purification.