Hydrothermal carbonization is a convenient way to convert biomass at rather moderate conditions into carbonaceous nanostructures, here, mesoporous network structures. A structural view on the micro- and nanoscale reveals interesting features defining the usefulness and application possibilities of the resulting carbonaceous materials as supports and for sorption purposes. Whereas weakly connected plant tissues result in good yields of carbon nanoparticles with very small sizes where the porosity is mainly interstitial, “hard” plant tissue is structurally transformed into a carbon replica with a rather well-defined, bicontinuous mesopore structure.