The use of constructed wetlands (CWs) for the treatment of the runoff waters from field cultivation and peat production in boreal climate is reviewed on the base of the authors' own research experiences and a literature study. The main water purifying mechanisms are presented, and the effectiveness of different types of CWs as well as the design and dimensioning parameters at the bottom of the effectiveness are discussed. In our view, CW area in relation to its catchment and hydraulically efficient CW shape are the main issues that a CW designer should focus on. This is because water purifying processes need time for efficient functioning, and both of these factors increase the actual residence time of water in a CW. Although both these factors stand up to as general guidelines, drawing conclusions from direct comparison of CWs in different locations is often misleading because of the differences in climate, catchment properties etc that highly effect the retention performance. Landscape improvement and increased birdlife are raised as examples of the ancillary benefits of CWs. The potential significance of CWs in water protection is dealt with at catchment scale. Comprehensive catchment management with variety of pollution control measures is needed if sustainable water quality improvement is aspired. When designed and located appropriately, CWs can make a useful contribution to this.