Offshore wind farms are typically designed with an anticipated service life of 25 years. Earlier it was assumed that the internal side of foundations below the lower working platform was airtight. If airtight corrosion was assumed to cease when the oxygen present inside the foundation was consumed. This assumption has been shown not to be fully valid as both seawater and oxygen have access to the inside of the monopile under certain conditions not the least on sites where large tidal variations exists (up to approx.. 10 m). This may result in active corrosion which can compromise the durability of the wind farm and reduce the service life if not prevented.DONG Energy undertake condition assessment of their wind farm foundations (+850 foundations altogether) on a regular basis. The monitoring comprises visual inspections corrosion measurements and evaluation of the corrosion environment (water quality). One mitigation measure where premature corrosion is seen is the use of cathodic protection.This paper presents details of the design of an internal cathodic protection system taking acidification and health risks into account. Also the commissioning and first performance data will be presented for one site.