Invasive squamous carcinoma of the cervix is preceded by a series of premalignant changes described as mild, moderate, or severe dysplasia, and carcinoma in situ. These premalignant states are identified by cervical cytology, diagnosed by colposcopy and if effectively treated, can prevent invasive squamous carcinoma of the cervix. Because of the apparent biological variation of the premalignant states, even the most aggressive cervical screening program cannot be expected to eliminate all invasive squamous cancer of the cervix. Optimal results of a cervical screening program will be achieved when all women under 35 years of age and sexually active have an annual cytological smear; the cytology is screened by a laboratory with high quality control; the patient's positive cytology is accurately assessed by an experienced colposcopist, and the premalignant lesion is effectively treated.