Measurement of DNA content was one of the first applications to be developed in the use of flow cytometry and is still used routinely in many experimental and, to a lesser extent, clinical studies. The goal of this technique is to produce a high quality DNA profiles for accurate analysis of DNA content and cell cycle distribution. In this chapter, we describe three DNA measurement methods that satisfy this requirement in different situations. It is widely accepted that the Vindelov method produces the highest quality DNA profiles in nuclei from solid tumours or cell lines. However, in many situations, DNA content is combined with another marker, so we describe a method which produces high quality DNA profiles in intact cells. Third, because the Vindelov technique requires prompt processing of fresh tumours, so we also describe a technique that derives nuclei from ethanol fixed tumours providing the convenience of storage before processing.