A range of different indices are available for assessing the health of trees in forests. An even larger range can be used for the assessment of the health of forest ecosystem . Most studies made in connection with "forest decline" and the impact of air pollution and other environmental stresses on forests have concentrated on the as essment of crown transparency and crown discoloration in individual trees. These are non- pecific indicators which are now known to be sometimes of relatively little value when determining the health of a forest ecosystem. Numerous problems exist with both, and the standardisation of asse sment between and even within countries has not been achieved. Consequently, tudie claiming to compare "defoliation" between different countrie cannot be ub tantiated. The emphasis on crown transparency and crown discoloration ha re ulted in the neglect of a number of other indices that could be of con iderable value. These include a variety of visual measures of crown condition and al o everal non-visual bioindicators. Some of these techniques are objective reducing the pre ent reliance on observer standardisation. A large number of potential technique are currently at the re earch tage and have yet to be adequately te ted in field trial . This repre ents an area where a substantial amount of further research is required.