Breast cancer is the most common malignant tumor in women: almost 10 % will suffer from breast cancer during their life and almost half of these will die of it. The spectrum of radiologic methods for diagnosing breast cancer is wide, including X-ray mammography, ultrasound, magnetic resonance mammography, and minimally invasive biopsies. After long-lasting controversies, breast cancer screening using X-ray mammography has now been introduced in Germany, following the projects in the Netherlands and Sweden. However, assessing mammographic films under screening conditions requires skills distinctly different from those needed under clinical conditions. This first part of two covers the histopathological basics and X-ray mammography; the second will deal with ultrasound of the breast and magnetic resonance mammography.