A relative and an absolute increase in the incidence of adenocarcinoma of the uterine cervix has occurred in the United States since 1970. Currently, most pathologists recognize the histologic and cytologic features of invasive adenocarcinoma of the cervix, but there is confusion surrounding the histologic features and biologic behavior of adenocarcinoma in situ, endocervical glandular dysplasia, and the definition of microinvasive adenocarcinoma of the cervix. Similarly, the distinction of in situ adenocarcinoma from an early invasive adenocarcinoma of the cervix may be problematic. This article focuses on the histologic criteria, biologic behavior, and some approaches to therapy for these challenging lesions. General conclusions based largely on published studies include the following: 1) adenocarcinoma in situ (AIS) is a recognizable precursor to invasive adenocarcinoma and can be divided according to distinct histologic subtypes; 2) AIS is multifocal or involves multiple quadrants of the cervix in about half of cases; 3) AIS can be cured by simple hysterectomy and in many cases may be treated effectively by cone biopsy; 4) endocervical glandular dysplasia is not a reproducibly recognizable lesion, and its behavior and existence are undefined; 5) criteria exist to permit the distinction of early invasive adenocarcinoma from AIS in about 80% of cases; 6) microinvasive adenocarcinoma of the cervix is complicated by the presence of multiple definitions; clinical decision making is best guided by assessment and reporting of the depth, horizontal extent, and presence of lymphatic or vascular invasion.