Three pathways of antigen recognition and activation converge in a common final pathway designed to permeabilize micro-organism invaders by way of a protein C5 to C9 protein complex that forms a pore in microbial lipid membranes. The pathways of activation are the classical, alternative, and mannose-binding lectin pathways (MBL). C1, C4, and C2 are early classical complement components. Factors D, B, are components of the alternative pathway. C3 and C5 to C9 are shared components. Complement regulatory proteins include factor I, factor H, C4-binding protein, and properdin. The proteins CD59 and DAF are also regulatory proteins whose deficiency is associated with paroxysmal nocturnal hematuria and are not described here. The C1 inhibitor protein (C1-INH) deficiency is associated with hereditary angioedema and is described in Chap. 79. There are multiple causes for secondary deficiencies of C3, for example, deficiency of factors H and I. Deficiency of some complement receptors are also implicated in leukocyte adhesion deficiency (LAD) and will not be discussed in this chapter.