This book analyzes the concept of using oil tankers' free cargo space and/or segregated ballast tanks to transport freshwater on the return leg, i.e. from oil unloading port back to oil loading port - also called freshwater backhauling (FWBH). The hypothesis considered is that by shipping freshwater this way to arid, oil exporting regions one can achieve a low cost and low GHG emission water supply system.The book analyzes the concept in a holistic manner, considering technical issues, transport and infrastructure costs, environmental impacts and contractual and legal issues.Costs are estimated both on a general and scenario specific level. It is found that freshwater could be shipped by way of backhauling by oil tankers to Saudi Arabia from Japan at a cost of between 0.83 and 1.16 USD/ton, including all infrastructure except distribution systems and excluding modifications to the tankers themselves.This cost level makes FWBH to a certain degree competitive with the chief water supply technology in the Middle East, desalination.It is clear, that in many cases FWBH is the superior option compared to desalination, especially when including environmental aspects.