The use of cytology for the detection and diagnosis of cancer has been a rapidly developing area of pathology. Cytology has been used for mass screening of asymptomatic people, for selective screening of individuals at high risk, and for diagnosis in patients suspected of having cancer. The only area in which mass screening has proved to be cost effective in reducing morbidity and mortality rates is cancer of the uterine cervix. Selective high risk screening has been proposed for endometrium, lung, urinary tract, gastrointestinal tract and oral cavity. Field trials have been performed for each site and some are still operational While a degree of success has been achieved in identifying patients with early and asymptomatic cancer, cost effectiveness has not as yet been demonstrated and it seems unlikely that it will be. The cytologic diagnosis of cancer of these sites in symptomatic patients is highly effective for lung and bladder cancer. It has been somewhat less effective for gastrointestinal cancer. It is also of value in identifying cancer cells in effusions and for cancer staging. Fine‐needle aspiration biopsy for the diagnosis of cancer has been used for over half a century, but in more recent years it has gained wider usage and is now used to diagnose tumors of almost any organ. It is highly cost effective, does not require hospitalization or anesthetic, and is rapid and accurate. Some of the indications and limitations of cellular diagnosis of cancer are presented.