I will describe a compactification of the moduli space of instantons in gauge theory on the union of two copies of Euclidean space R^4 intersecting transversely in R^8, and its orbifolds. The theory of qq-characters is an application. The qq-characters are the deformations of the q-characters of quantum affine algebras, which, in turn, are the deformation of the ordinary Kac-Moody characters