Global economies are confronting major energy challenges. Mitigating the energy depletion crisis and finding alternative and unconventional energy sources have been subjects for many investigations. Plant-sourced biomasses have started to attract global attention as a renewable energy source. Maximizing the performance of the biomass feedstock in different applications requires the availability of reliable and cost-effective techniques for characterization of the biomass. Comprehending the structure of lignocellulosic biomass is a very important way to assess the feasibility of bond formation and functionalization, structural architecture, and hence sculpting of the corresponding structure−property liaison. Over the past decades, non-invasive techniques have brought many pros that make them a valuable tool in depicting the structure of lignocellulosic materials. The current chapter will be focused on the applications of Fourier transform infrared (FTIR) spectroscopy especially in the mid-infrared region in the compositional and structural analysis of lignocellulosic biomasses. The chapter will provide a display of examples from the literature for the application of FTIR spectroscopy in finding the composition of various biomasses obtained from different parts of plants and applied for wastewater treatment. A comparison between biomasses and physically/chemically treated products will be discussed.