SYNOPSISThe changes occurring at the periphery of invasive cervical carcinomas have been studied in detail, with particular reference to the incidence of preinvasive carcinoma. The various criteria used in the cytological diagnosis of malignancy have been evaluated, and the implications of large cytological screening programmes are discussed in a brief review of the literature.In 1867 Beale illustrated exfoliated malignant epithelial cells while the entity of preinvasive carcinoma was probably first recognized by Williams (1888) and more clearly defined by Schauenstein (1908) although the term 'carcinoma in situ' was not introduced until 1932 by Broders. Exfoliative cytology was finally established as a routine diagnostic method in this field by the work of Papanicolaou and Traut (1943) and has been actively practised since then all over the world. Although Schottlaender and Kermauner (1912) showed histologically that precancerous changes were present at the margins of invasive carcinomas the validity of the concept of preinvasive carcinoma depends ultimately on clinical evidence (Petersen, 1955;Koss, Stewart, Foote, Jordan, Bader, and Day, 1963).The term 'preinvasive carcinoma' is used here as synonymous with 'carcinoma in situ' but a more recent concept termed 'dysplasia' has been introduced and currently finds great favour with American workers. The boundaries of the latter condition are ill-defined and the prognosis is uncertain but a proportion develop into preinvasive carcinoma (Stern, 1959) while many examples show regression