The talk is a brief review of structure of quantum effective action in six-dimensional gauge theories. The $N=(1,0)$ supersymmetric theories are constructed in harmonic superspace. Aspects of maximally extended global 6D super Yang-Mills theory are discussed. Manifestly $N=(1,0)$ supersymmetric and gauge invariant quantum effective action is constructed and results of one-and two-loop calculations of its divergences are presented.