Biosorption is an efficient technology that is widely used globally to eliminate numerous pollutants from wastewater. Using this approach, various biosorbents have been identified, tested, and used to remove hazardous pollutants such as heavy metals, dyes, pharmaceutical compounds, and radionuclides at both the lab and industrial levels. Biosorbents are one of the pillars of bioremediation because they have a synergetic impact due to their efficient adsorption ability for eliminating toxic materials, abundance, and ability to minimize waste, which helps mitigate environmental and ecological problems. In recent decades, a wide variety of novel biosorbents, including bacteria, algae, fungi, etc., have been identified and employed in environmental remediation. Although many biological species have been investigated as adsorbents for separating pollutants and for their removal efficiency and economical efficacy, no major breakthrough have been achieved in practical applications. Hence, researchers have paid increasing attention to developing selective, surface-modified, high-performance, low-cost biosorbents. Characterization of such biosorbents is essential in understanding their physical, chemical, and mechanical properties and dominant sorption mechanism. Development and application of biosorbents are accelerated by their extensive characterization and accomplishments in this field. The biosorbents selected for investigation were characterized by quantitative and qualitative analysis to determine the particle size, surface area, pore size and volume, surface morphology, and functional group; the process included elemental analysis, crystallite analysis, magnetic intensity analysis, magnetic property analysis, etc. This chapter summarizes the characterization techniques currently employed to investigate biosorbents and provides brief descriptions of their principles and operation, selection of an appropriate strategy, and advantages based on physicochemical properties. © 2023 Elsevier B.V., All rights reserved.