Low-grade peritoneal serous carcinomas have been the subject of limited study, and their distinction from peritoneal serous psammocarcinomas and serous borderline tumors is not always easy. The clinicopathologic features of 14 low-grade serous carcinomas, 7 psammocarcinomas, and 19 serous borderline tumors of peritoneal origin were compared. Average ages were 58 years (low-grade serous carcinomas), 48 years (borderline tumors), and 40 years (psammocarcinomas). Typical clinical presentations were abdominal pain, abdominal mass, or both, with the tumors incidental in 37% (borderline tumors), 43% (psammocarcinomas), and 36% (low-grade serous carcinoma). Operative and gross findings varied from nodules to adhesions to a dominant mass. Treatment was surgical debulking in most cases, with biopsy alone for eight borderline tumors. Seven patients with low-grade serous carcinoma were alive when last seen, but follow-up duration is short (average, 1.2 years): five were without disease, one had recurrent disease and one persistent disease. One patient with serous carcinoma died of disease at 3.5 years, and two patients died of other causes. Three patients with psammocarcinoma were alive without disease (average 3.3 years). Fourteen patients with borderline tumors were alive (average 3 years): 10 were without disease, 2 had persistent disease, and serous carcinoma developed in 2. The low-grade serous carcinomas resembled the invasive implants of ovarian serous borderline tumors. lacked high-grade nuclear atypia, showed tissue, lymphovascular space invasion, or both and had appreciable solid epithelial proliferation. Some serous carcinomas showed abundant psammomatous calcification suggesting psammocarcinoma but had too much epithelial proliferation for that diagnosis. The psammocarcinomas showed at least 75% psammoma bodies, no more than moderate cytological atypia, tissue or lymphovascular space invasion, or both, and rare epithelial proliferation less than 15 cells across. Adequate sampling was necessary to identify invasion, with highest yields of invasive foci in omental samples; individual foci in some cases of carcinoma resembled borderline tumor. The serous borderline tumors resembled the noninvasive implants of ovarian serous borderline tumors, lacked invasion, and did not show nuclear atypia of the degree seen in grade 2 or grade 3 serous carcinoma. Low-grade serous carcinoma, psammocarcinoma, and serous borderline tumors of peritoneal origin share some clinicopathologic features and may be underrecognized at surgery and gross examination. Because of overlapping microscopic patterns, adequate sampling is mandatory to identify small foci of invasion that exclude a borderline tumor and identify significant cellularity that excludes a psammocarcinoma. Conservative therapy is merited for younger women with borderline tumors. Maximum debulking is recommended for bulky symptomatic borderline tumors, low-grade serous carcinoma, and psammocarcinoma. Although short-term outcomes for the carcinomas appear favorable, follow-up is too limited to determine long-term outcomes.