Complement-dependent cytotoxicity is one of the mechanisms by which therapeutic monoclonal antibodies are successful against cancer. Complement is one of the innate immune defence systems, whose activation products and membrane-bound regulators interact with cells of the adaptive immune response. The complement system is currently undergoing a re-appreciation in its role within the immune surveillance of tumour. The majority of human tumours are low immunogenic. Complement may be involved through direct, 'danger signal'-elicited activation or via infiltration of inflammatory cells, which express complement components. Inflammatory cells may be associated with malignant transformation and tumour regression. The evidence for the effects of complement activation and regulation on tumour progression and expansion will be reviewed using in vivo, in vitro, and patient studies, and conclusions drawn for the implications in therapy and management of tumour patients.