Slow pyrolysis, fast pyrolysis and gasification are thermochemical processes to produce fuel and chemicals from biomass. The char co-products from these processes have much potential as biochars: sustainably produced biomass charcoals used for amending soils and sequestering carbon. As biochar properties vary significantly with feedstock and reaction conditions, biochar characterization is critical for understanding these variations, for obtaining meaningful data from biochar agronomic studies, and for determining the most beneficial and economical application of a given char co-product (fuel, sorbent or soil amendment). This dissertation describes biochar characterization methods used to relate thermochemical reaction conditions such as temperature, heating rate and reaction atmosphere to biochar properties. These methods include proximate analysis, CHNS elemental analysis, BET surface area gas sorption analysis, helium pycnometry, scanning electron microscopy (SEM), X-ray fluorescence spectroscopy (XRF), FT infrared spectroscopy with photoacoustic detection (FTIR-PAS), advanced solid-state 13C nuclear magnetic resonance spectroscopy (NMR), cation exchange capacity (CEC), pH, electrical conductivity, and short-term soil incubations with soil respiration and water retention measurements. One outcome of this research has been the incorporation of biochars derived from corn stover and switchgrass, and derived by fast pyrolysis and gasification, in the wider biochar community discussions.