Immunoglobulins (Igs) play key roles in immune protection of mucosal surfaces that form an immense and vulnerable interface with the environment. In humans, the principal Ig in most but not all mucosal secretions is IgA. In some secretions, IgG and IgM contribute toward immune defense. IgA in secretions comprises polymeric (chiefly dimeric) forms of IgA made up of two monomers of IgA, each made up of two heavy or alpha chains and two light chains, linked together via J or joining chain, and covalently associated with secretory component, a polypeptide derived from the receptor responsible for transporting IgA into the sections. The chapter provides an overview of these mucosal Ig components, addressing their evolution, interspecies differences, genes, expression, structural features, and glycosylation.