Skeletal transformations of heteroaromatic compounds while cleaving their endocyclic bonds have been emerging as new synthetic tools in modern organic chemistry. Especially, endocyclic insertion of a heteroatom into a heteroaromatic core is a game-changing approach to exotic classes of heterocyclic compounds. In this Highlight Review is summarized recent progress of ring-expanding and ring-opening reactions of benzofurans and indoles.