Corrosion of main cables on suspension bridges is a major problem on a world-wide basis. The load carrying wires are covered by wrapping wire and corrosion is therefore hidden and often progresses to a very serious level before it is detected. Many examples have illustrated that traditional corrosion protection systems for main cables do not prevent corrosion, but merely slows it down. This has led to the development and application of dehumidification of main cables, which truly prevents corrosion. A dehumidification system blows dry air through the main cables and keeps the atmosphere in the cables so dry that corrosion can not occur. This paper describes the details of such a system, design considerations, results of life cycle cost analysis and experience from systems in operation