This paper will review the methods that have been advanced for the estimation of parameters of models quantifying the population characteristics of the kinetic behavior of endogenous and exogenous substances in individuals of the population. Such methods are used frequently, for example, in pharmacokinetic studies. In certain populations, especially biological ones, considerable kinetic variability between population members is present. The models with which we are concerned describe this variability. Some interindividual kinetic variability may be explainable on the basis of measureable concomitant variables, but much of it may remain unexplainable on such a basis. We give this matter particular attention. The assumptions underlying the models are critically discussed.