The theory of the sick lobe states that breast carcinoma is a lobar disease developing most often within a single lobe, meaning that, at an early stage, breast carcinoma occupies a limited, anatomically well-defined portion of the breast. This theory unites observed patterns from the genetic, developmental and morphological perspectives, into an overall concept. Breast Cancer: A Lobar Disease, presents this hypothesis and its consequences. The body of evidence, pro and contra, generated in recent years will be presented in this volume. The chapters, all authored by leading experts in their respective areas, gather evidence from the perspectives of epidemiology, genetics, radiology, anatomy, developmental biology, morphology, endoscopy, ultrasound and therapeutics to give the reader a full picture of recent developments regarding the sick lobe hypothesis.