This Bulletin reviews the evidence for a suggested forest effect in the acidification of surface waters in Great Britain. Acid deposition from the atmosphere within susceptible areas of Britain has affected fresh water flora and fauna, causing the decline and in some instances the complete loss of fish populations. Currently there is a debate about whether the presence of forests has increased the acidity of surface waters and contributed to the observed decline. The evidence for the significance and scale of such a forest effect is by no means clear and only limited conclusions can be drawn from studies undertaken so far. Long-term monitoring of streamwater chemistry within individual catchments is being undertaken. In due course the results of these studies will allow researchers to come to firm conclusions regarding the extent of any forest acidification effect.