Abstract：Optical responses of atomically thin 2D materials are greatly influenced by electron-hole interactions. It is by far established that exciton signatures can be well-identified in the optical absorption spectrum of quasi-2D materials. However, the same level of understanding of excitonic effects on nonlinear optical responses and the ability to compute them accurately is still much desired. We first show how excitonic effects in nonlinear optical response such as shift current and second harmonic generation can be studied with a time-dependent density matrix calculation. For shift current response, we show great enhancement from excitonic effects on optical matrix elements in monolayer GeS and BN nanotubes. In contrast, excitonic enhancement in SHG spectrum is seen in the lowest exciton resonance in monolayer h-BN but not in MoS2. In the second part of the presentation, based on the functional integral formalism and working in the velocity gauge, we derive another expression for second-order optical responses by dressing electron-photon interactions with electron-hole ladder diagrams. The discrepancy between the two expressions will be discussed.