The Bethesda System for reporting cervical/vaginal diagnoses was introduced to replace the numerical Papanicolaou class designations, thereby facilitating precise communication between cytopathologist and clinician. The terminology for squamous epithelial lesions includes the following categories: 1) atypical squamous cells of undetermined significance; 2) squamous intraepithelial lesion (SIL), which encompasses the spectrum of squamous cell carcinoma precursors, divided into low-grade SIL (human papillomavirus [HPV]-associated cellular changes, mild dysplasia, and cervical intraepithelial neoplasia [CIN] I) and high-grade SIL (moderate dysplasia, severe dysplasia, and carcinoma in situ and CIN II and III); and 3) squamous cell carcinoma. The rationale for including HPV-related changes (koilocytosis) with CIN I within low-grade SIL is based on the morphologic, behavioral, and virologic similarity of these two lesions, which precludes their separation in a consistent and reliable fashion. For the same reasons, CIN II and III lesions have been combined within the category of high-grade SIL. The term "atypical squamous cells of undetermined significance" is used for cytologic findings that do not fulfill the criteria for defined benign reactive changes or SIL. Therefore, this term is more restricted in usage as compared with the wide range of interpretations previously ascribed to "atypia" or "inflammatory atypia."