The morphoedaphic index (MEI), a fish yield estimator, was historically reviewed and evaluated. Background papers leading to the development of the MEI were discussed, as well as more recent advances. The MEI was defined and its criteria for use examined. The significance of its components — total dissolved solids and mean depth — were explained. The MEI was compared with related models developed by other workers. The global range of the MEI was outlined as well as restrictions on its use. Sustained fish yields at different MEI values for various climatic regions were predicted. Several special applications of the MEI were described, including: prediction of angling yield in north-temperate lakes; prediction of commercial fish yield on a newly created reservoir; indicator of ecological stresses and changing environments; and contrasting responses to cultural eutrophication in the littoral and profundal zones of lakes. The implications of the MEI for ecological theory were explored, and its potential for future application in management was outlined.