Mannan-binding lectin (MBL) is a plasma protein found in association with several serine proteases (MASPs) forming the MBL complex. MBL recognises carbohydrate structures arranged in a particular geometry, such as those found on the surface of micro-organisms. When bound to e.g. bacteria the MBL complex will initiate the activation of the complement cascade. Mounting evidence supports the importance of the MBL pathway of complement activation in innate immunity. In this review, we focus on the structure and function of the proteins within the MBL pathway and address the properties of the pathway as an initiator of the host response against potential pathogenic micro-organisms.