Macromolecular structure is of critical importance to the reactivity and persistence of biochar. This chapter discusses pyrolysis conditions and chemical transformations of biomass that produce a continuum of macromolecular structures. The review synthesizes data on molecular dimensions, heteroatoms, and free radicals, and briefly describes the spectroscopic techniques used to measure them. Fundamental principles of aromaticity and the nature of non-aromatic domains in biochars are reviewed, and examples are given of functionality and reactivity associated with macromolecular characteristics.