  We systematically analyze the effective action on the moduli space of (2,0)
superconformal field theories in six dimensions, as well as their toroidal
compactification to maximally supersymmetric Yang-Mills theories in five and
four dimensions. We present a streamlined approach to non-renormalization
theorems that constrain this effective action. The first several orders in its
derivative expansion are determined by a one-loop calculation in
five-dimensional Yang-Mills theory. This fixes the leading higher-derivative
operators that describe the renormalization group flow into theories residing
at singular points on the moduli space of the compactified (2,0) theories. This
understanding allows us to compute the a-type Weyl anomaly for all (2,0)
superconformal theories. We show that it decreases along every renormalization
group flow that preserves (2,0) supersymmetry, thereby establishing the
a-theorem for this class of theories. Along the way, we encounter various
field-theoretic arguments for the ADE classification of (2,0) theories.
