The polymerase chain reaction (PCR) is an extremely sensitive method owing to the repetitive multiplication of template molecules. This property is a drawback for quantitative measurements because small differences in the multiplication factor lead to large differences in the amount of product. Two methods can be used to solve the problem of quantification: kinetic methods based on the determination or comparison of the amplification factor; and coamplification methods, which compare the amount of product to that of a simultaneously amplified standard template. An overview of the theoretical background of both methods is presented. For selection of a suitable method, both theoretical and practical considerations are important. Kinetic methods are the most convenient if PCR can be performed without opening the tubes, as in some apparatus using fluorescence detection. Coamplification methods can be done without expensive equipment but requires the parallel running of several PCR tubes. When the number of initial template molecules is close to one, as in the limiting dilution technique, statistical considerations become important.