AbstractThe history of loss assessment over the past 100 years is indicated, and the three principles: detection, measurement, and evaluation, are discussed. Economic and political pressures now demand objective justification for widely used methods of loss control, and the trend is towards a clear indication of the opportunities for gain which occur when harmful organisms are efficiently controlled. New emphasis is being given to predicting the need for controls. Accurate loss assessments are expensive in terms of money and trained staff, and the most important single factor in efficient investment is an unambiguous statement of aims so that full advantage can be taken of the economies offered by modern methods.