This chapter summarises all aspects relevant to understanding residual biomass in the context of bioeconomic processing solutions. Initially, the origin of residual biomass is discussed, starting at the farm level and extending to industrial processing. Factors influencing the quality aspects of residual biomass are key to understanding the suitability of valorisation pathways and its potential as a relevant biomass for the circular and sustainable bioeconomy. Many bio-based products are the result of a combination or integration of fractions of a raw biomass, so this chapter provides an in-depth understanding of the major and minor fractions of plant biomass and links these fractions to potential applications of high commercial interest. Different discourses allow a deep dive into relevant aspects that are key for a thorough understanding of the full valorisation potential of residual biomass.