One most important global challenge is achieving the balance between food production and the increasing global population. The main measures are to improve the crop yields and reduce the economic risk caused by agricultural uncertainty. Therefore, the real-time monitoring of crops and soils become one important part in agricultural production, which can be used to assist farmers for appropriate farming decisions. Optical spectroscopic methods have been widely utilized for agriculture, relying on their superior abilities of non-destructive, compact, rapid and real-time monitoring, which plays an important role for the rapid development of precision agriculture. The goal of this work is to give an overview of research advances in agricultural applications with various spectroscopic techniques, including visible and near-infrared (Vis–NIR) spectroscopy, fluorescence spectroscopy, Raman spectroscopy, laser induced breakdown spectroscopy (LIBS), and hyperspectral imaging (HSI), for acquiring agricultural indicators such as leaf chlorophyll content (LCC), leaf nitrogen content (LNC), leaf water content (LWC), soil humidity, soil organic matter (SOM), and nutrient elements. Meanwhile, we discussed how the quantitative data derived from spectroscopic methods provide valuable and decision-supporting information for the agricultural production, and then how to use these information to manage the practical farming. In the end, we give some viewpoints on the current challenges and future perspectives of optical spectroscopy in agriculture activities.