About 15-20% of all ovarian neoplasms are of borderline type (or atypical proliferative or carcinoma of low malignant potential). They represent a common diagnostic and treatment problem both for the pathologist and for clinicians. The borderline tumors occur most commonly in childbearing age, show an indolent course and have good prognosis but are resistant to the traditional chemotherapies. The serous borderline tumors are the most common types of borderline ovarian tumors and they can cause differential diagnostic problems even for the experienced pathologist. We studied 30 cases which were diagnosed in our institute from 2000 to 2008. Thirteen were typical serous borderline tumors, in 7 cases the pattern was micropapillary, in 2 cases with microinvasion and in the remaining 8 cases the borderline tumors were associated with low-grade serous carcinomas. Seventeen of the 22 borderline cases were stage I tumors. There were noninvasive implants in the remaining 5 cases and in the cases of the low-grade carcinomas we could find, besides the noninvasive implants (in 3 cases), invasive implants or metastasis too. The main diagnostic problems in serous ovarian borderline tumors are the presence of micropapillary pattern, to detect microinvasion, or to differentiate the pseudo-borderline pattern of the low-grade serous tumors from a real borderline tumor and especially to diagnose the extraovarian diseases (types of implants). We discuss these diagnostic problems and criteria according to recent literature and our experience.