Co-selection is a term used to denote the mutual positive selection of individual members from within two diverse populations, such that selection of members within one population is dependent on interaction with (recognition of) one or more member(s) within the other population. Co-selection is a recurring theme of the idiotypic network model that my colleagues and I have developed. This paper discusses the role that co-selection plays in basic symmetrical network theory and in a network model that resolves the I-J paradox. It proposes that co-selection of helper T cells and HIV variants plays a role in the pathogenesis of AIDS. The AIDS model involves a role for the T cell receptor in the infection of T cells. Finally, a way in which a co-selection process may potentially be used in the prevention and therapy of harmful forms of immunity is described.