An increasing global population and current societal reliance on fossil fuel resources has precipitated an ongoing crisis at the energy-materials-environment nexus. Sustainable resources are required to meet the demands of developed and emerging economies while mitigating climate change and protecting the biosphere; this has promoted the "zero-waste economy" and "trash-to-treasure strategy" concepts. [1] Achieving such ambitious goals necessitates a transition from fossil resources to renewables such as nuclear, solar, wind, and wave power, and biomass. Lignocellulosic biomass, such as agricultural and forestry residues, crops, and wood, is the most abundant (inedible to humans) biomass resource and a potential low-carbon feedstock to partially replace fossil carbon. It comprises cellulose and hemicellulose carbohydrate polymers embedded in a lignin matrix. [2] Lignin is a three-dimensional network of polyaromatic alcohols, whose upgrading can offer routes to renewable valueadded chemicals. [3] Hemicellulose comprises pentose (predominantly xylose) and hexose units connected by glycosidic bonds. Cellulose is a water insoluble, linear polysaccharide formed from glucose units linked via b-1,4-glycosidic bonds. [4] As an important renewable feedstock, biomass, and its conversion to materials, chemicals, and fuels is intensively researched. [5] Biomass conversion may occur through non-catalytic (thermo)chemical routes, e.g., pyrolysis and hydrothermal processing, and/or catalytic (bio)chemical routes, e.g., homogeneous, heterogeneous, and enzymatic catalyzed processes [3,6] (Figure 1). Bio/hydrochars are an important class of solid materials obtainable from biomass due to their applications as adsorbents and catalysts, and as precursors to activated carbons and other functional materials. [7] Bio/hydrochars and activated carbons are all pyrogenic materials, i.e., produced by thermochemical conversion and containing some organic carbon. Activated carbons may derive from any carbon source (fossil, waste, or renewable) and irrespective of sustainability concerns, whereas biochars are (now) viewed as pyrogenic materials derived from sustainably sourced biomass and not used as a fuel. [8] Activated carbons are amorphous and non-graphitic, with high surface areas and (usually) microporosity and a small proportion of hydrogen or oxygen, whereas biochars, and particularly hydrochars, often possess a higher oxygen content. A few studies refer to biochars as pyrochars (being derived by pyrolysis of biomass) and/or conflate the terms pyrochar and hydrochar under the banner of biochars. [9] In this review, we follow widely adopted literature conventions for char definitions [7c,10] : biochar is a carbon-rich, porous solid produced by biomass pyrolysis under restricted oxygen and moderate temperatures (350-700°C), whereas