Mannose-binding lectin (MBL) is a plasma protein with an important role in the innate immune system. MBL recognizes pathogens through carbohydrate structures present on the surface of a range of pathogenic organisms including viruses, bacteria, fungi and protozoans. These structures may be referred to as pathogen-associated molecular patterns (PAMPs). After binding to PAMPs, MBL promotes C1- and antibody-independent activation of complement, leading to complement-mediated killing and/or phagocytosis. MBL is also known to modulate the secretion of cytokines from macrophages and to mediate the clearance of apoptotic cells as such playing a role in the inflammatory response. This book summarizes the actual understanding of human MBL biology and introduces the general aspects of the structure, function and genetics of MBL, as well as an analysis of the role of MBL in the predisposition to clinically relevant diseases.