“Approximately 1.6 million women in the United States have breast biopsies each year. The accuracy of pathologists’ diagnoses is an important and inadequately studied area. Although nearly one-quarter of biopsies demonstrate invasive breast cancer, the majority are categorized by pathologists according to a diagnostic spectrum ranging from benign to preinvasive disease. Breast lesions with atypia or ductal carcinoma in situ (DCIS) are associated with significantly higher risks of subsequent invasive carcinoma, and women with these findings may require additional surveillance, prevention, or treatment to reduce their risks. The incidence of atypical ductal hyperplasia (atypia) and DCIS breast lesions has increased over the past 3 decades as a result of widespread mammography screening. Misclassification of breast lesions may contribute to either overtreatment or undertreatment of lesions identified during breast screening. The pathological diagnosis of a breast biopsy is usually considered the gold standard for patient management and research outcomes. However, a continuum of histologic features exists from benign to atypical to malignant on which diagnostic boundaries are imposed. Although criteria for these diagnostic categories are established, whether they are uniformly applied is unclear. Nonetheless, patients and their clinicians need a specific diagnostic classification of biopsy specimens to understand whether increased risk for breast cancer exists and how best to manage identified lesions. Although studies from the 1990s demonstrated challenges encountered by pathologists in agreeing on the diagnoses of atypia and DCIS, the extent to which these challenges persist is unclear. These issues are particularly important in the 21st century because millions of breast biopsies are performed annually. For these reasons, we investigated the magnitude of overinterpretation and underinterpretation of breast biopsies among a national sample of practicing US pathologists in the Breast Pathology (B-Path) study. We also evaluated whether patient and pathologist characteristics were associated with a higher prevalence of inaccurate interpretations.”