Since their initial description, mucinous cystic neoplasms have been difficult to classify. This article attempts to clarify histological, clinical, and genetic criteria so that the pathologist can categorize each mucinous cystic neoplasm into 1 of 4 possible categories. Mucinous cystadenomas contain a single layer of mucin-producing, columnar epithelium lacking significant atypia. Borderline mucinous cystic neoplasms contain cells with moderate atypia. Mucinous cystic neoplasms with in situ carcinoma show significant architectural and cytological atypia. When invasive carcinoma is present in association with a mucinous cystic neoplasm, then the diagnosis of invasive mucinous cystadenocarcinoma should be made. The categorization of mucinous cystic neoplasms into these groups is essential because it accurately predicts outcome, provided that the tumor has been sampled and examined thoroughly. Completely removed mucinous cystadenomas, borderline mucinous cystic neoplasms, and mucinous cystic neoplasms with in situ carcinoma follow benign courses. Partial resection should be avoided as evidence suggests that mucinous cystic neoplasms can progress from adenomas to borderline lesions to carcinomas in situ to invasive carcinomas over time; partial resection should be avoided if possible. Modern molecular genetic techniques are helping to unravel the origins of rare variants of mucinous cystic tumors, such as the mucinous cystic tumor with an associated osteoclast-like giant cell tumor and the mucinous cystic tumor with sarcomatous stroma.