Boreal peatlands are substantial terrestrial carbon storages. They are also important for the function of several large-scale cycles and systems that humanity benefits from. Drainage affects peatlands by establishing a new, lower equilibrium for the groundwater table. As a consequence of a lowered groundwater table and physical disturbance from the ditch, a chain of events is triggered, causing a number of changes in the hydrology and hydrochemistry of peatlands. The review aimed at gathering the most relevant literature on how boreal peatlands are affected by drainage. This was carried out by searching for scientific articles on online databases and libraries. Besides reviewing hydrology and hydrochemistry, a compilation of boreal peatland characteristics and a background on the history of Swedish peatland drainage are handled in the report. The results show that concentrations and loads for a long range of water quality parameters are affected by drainage. The effects vary a lot depending on different peatland characteristics. Both runoff and chemical changes depends largely on whether ditches reach mineral soil beneath the peat. The most obvious and adverse effect on chemistry is a large increase in suspended solid concentration and export. Other changes include increases of mineralized nitrogen, decreased organic carbon transport, increased annual runoff and increased levels for a number of elements, such as Mg, Ca, K and Mn.