The complement system is a crucial mediator of the innate immune mechanisms; its three major activation pathways contribute to cellular homeostasis, and protect the host from pathogens and self-derived components. The complement pathways are tightly regulated by surface-bound molecules and soluble proteins in order to avoid over-activation and consequent pathological inflammatory response. However, tumor cells have evolved several strategies to escape from complement-mediated elimination and create a conducive tumor microenvironment to promote its growth and metastasis. The excessive expression of complement proteins in malignant tumors and avoidance of membrane attack complex formation on its surface have been observed in almost all types of cancers. Complement cleavage products are found extensively deposited along the tumor vasculature, influencing the immunosuppressive tumor microenvironment. This chapter discusses the role of complement effectors and regulators in cancer immunity.