Smartphone based optical sensing has recently attracted a lot of interests. The wide availability of smartphones and their advanced computational and wireless communicational capabilities coupled with chemical sensing abilities of optical techniques provide intriguing possibilities of “lab on a palm.” In this talk I will present our work on smartphone based optical spectroscopy. Compact spectrometers are developed by using a G-Fresnel device with the dual functionalities of focusing and wavelength dispersion, which can enable miniaturization of optical spectrometers and thus the integration of optical spectroscopy with smartphone technology. The fabrication and characterizations of the smartphone based spectrometer are presented. I will discuss potential applications of smartphone based optical spectroscopy. Specifically, the use of smartphone based diffuse reflectance spectroscopy for hemoglobin sensing and for detecting plant diseases will be discussed.