Suitable sites for tidal power generation are mostly situated in estuaries, where with a high tidal range of 5–15 m, the construction of a barrage is proposed to impound water during tidal rise. Electricity is then generated using turbines exploiting the difference in level as the tide drops outside the impounded area. Most sites of this kind contain complex and productive ecological systems. Their importance, for example, for the migrant and over-wintering waterfowl usually supported by these estuaries, can be at an international level. The impoundment of water behind a barrage can submerge intertidal flats used as feeding areas by these waterfowl. Furthermore, by reducing tidal height, a barrage can affect the extent of saltmarshes which are not only important feeding grounds but which also have intrinsic interest for their unusual flora. The potential environmental impact of tidal power barrages can, therefore, be serious. The impoundment of an estuary can also create water quality problems, if there are sources of pollution feeding into the system. This may require major investment in effluent treatment before the project can proceed. The most important factor with respect to water quality change is the reduced tidal currents, following impoundment, acting with the additional retention time of water in the system to compound pollution problems. However, the problem may be reduced due to the larger mean volume of water in the impounded estuary. Further environmental effects of tidal power generation can include matters concerned with coast protection, flood control and groundwater. The creation of a large impounded area can lead to pressure for its development as a waterbased leisure facility. Leisure development, particularly the use of powered boats, can lead to conflict with nature conservation interests. The paper discusses all the above issues and, despite the clear fact that all the matters involved are clearly site-specific, some overall conclusions will nevertheless be drawn concerning the environmental effects of tidal power generation in relation to alternative methods of electricity generation.