Techniques which estimate the rate of aqueous flow generally require the use of tracer substances. Determination of the distribution of the tracer in the relevant body compartments permits calculation of the rate of flow within the limits of accuracy of the method used. The underlying theory, as well as the advantages and limitations of methods employing systemic, topical, intracameral, and intravitreal administration of tracer substances are reviewed. Since these methods all assume that the rate of aqueous secretion is constant, yet the presence of a diurnal rhythm of flow has been demonstrated in both rabbits and humans, a compartmental model of a circadian system based upon the vitreous depot technique is presented. This model estimates the degree to which a continuously changing rate of aqueous flow limits the ability to determine aqueous flow rate accurately by this particular method and illustrates this limitation, which is common to all tracer methods.