Various KLK proteins and their encoding genes have attracted increased attention among scientists and clinicians worldwide since they represent very interesting and functionally distinct biomarkers, particularly, in cancer. This book reviews the role of kallikrein-related peptidases (KLKs) in a wide range of cancers, including lung, prostate, breast and ovarian cancer. It provides clinicians, physician scientists and researchers with a comprehensive overview on the clinical relevance of KLK expression in various malignancies.