The study of idiotypes (Id) and the regulation of their expression has attracted a good deal of attention since Jerne enunciated the Id network hypothesis in 1973; 1 indeed, it makes up the theme of the present volume. In this chapter the following questions will be addressed: what is an Id? Is the expression of Id regulated independently of the expression of antibody levels? What is the function of such regulation in the immune response? What is the nature of the regulatory apparatus? And how do such regulatory elements specific for Id arise?