This chapter focuses on human antibody effector function. A molecular explanation of antibody effector function requires the description of multiple antibody molecules cross-linking an array of antigen molecules to multiple effector molecules. The antigen molecules are likely to be on a cell surface and the effector molecules are either large, as for complement, or on a cell surface, as for fragment crystallizable (Fc) receptors. The chapter presents crystal structures for (1) Fab fragments, (2) Fab fragments complexed with a number of antigens, and (3) Fc from immunoglobulin (IgG). It also has the low-resolution structures of two mutant whole IgG molecules. These mutant molecules lack the hinge region and generally show poor effector activity. They crystallize and show a complete diffraction pattern because the loss of the hinge reduces their flexibility. The crystal structures of human and rabbit Fc from IgG are determined to intermediate resolution and analyzed in detail in terms of potential interacting sites. The chapter discusses complement activation by antibodies. The complement cascade can be usefully divided into two phases: (1) the deposition of C3 on the antigenic cell surface, thereby facilitating phagocytosis, in the classical pathway this requires the sequential activation of C1 and C42, (2) the formation of the membrane attack complex resulting in cell lysis, this requires the activation of C5 and attachment of C6–9.