During the 1980's, environmental issues grew rapidly in importance, and more sophisticated modelling scenarios were developed as an aid in predicting the transport and persistence behaviour of pesticides in various ecosystems. In most of the pesticide transport models, two particular parameters are of prime importance: a parameter to describe the partitioning between soil and water, the soil-water partitioning coefficient (KJ; and a parameter to describe the time required for one-half of the applied pesticide to disappear (Tl/2) based on first-order kinetics, where T112 = 0.693/k, and-k is the first-order disappearance rate constant. Much of the discussion that follows deals with issues and techniques surrounding the use of these key parameters, and problems associated with accurately measuring and correctly using them.