Hyperarithmetic theory is the first step beyond classical recursion theory. It is the primary source of ideas and examples in higher recursion theory. It is also a crossroads for several areas of mathematical logic. In set theory it is an initial segment of Godel's L. In model theory, the least admissible set after. In descriptive set theory, the setting for effective arguments, many of which are developed below. It gives rise directly to metarecursion theory (Part B), and yields the simplest example of both arecursion theory (Part C) and Erecursion theory (Part D).