Some technical aspects of Papanicolaou staining are reviewed. The history of the technique is traced from Mallory's aniline blue technique through Masson's trichrome procedure to the techniques of Shorr. The histochemistry of the three Papanicolaou staining solutions (aluminum-"hematoxylin", OG and EA) is discussed. The structure of the aluminum-hematein chelate and its mode of action are considered. A tentative mechanism is proposed for the characteristic differential counterstaining produced by Papanicolaou techniques, i.e. orangeophilia vs cyanophilia. It is suggested that differential counterstaining occurs as a consequence of 1) differences in cytoplasmic density, 2) differences in the molecular size of the anionic dyes, and 3) the inhibitory effects of phosphotungstic acid on the binding of small dyes. The review considers some recent quantitative studies of Papanicolaou stained cells and outlines some modifications of Papanicolaou's procedures. The text concludes with a discussion of alternatives to the Papanicolaou technique.