Second-order nonlinear optical(NLO)crystalline materials are fundamentally and technologically important for their ability to double or triple the frequency of lasers.This article provides a brief review of the atom response theory(ART)of NLO responses recently developed on the basis of the partial response functional method.The ART analysis enables one to quantitatively evaluate the contributions of individual constituent atoms to the second harmonic generation(SHG)response of a NLO crystal material on the basis of first principles DFT calculations.The general partitioning principles developed in our recent work provide the conceptual foundation for determining the functional motifs of SHG responses.In this mini review we will focus on the concepts and principles as well as on applications with examples.Some practically important empirical rules resulting from the ART studies will also be reviewed.