**Abstract**

Biorefining produces technical lignins that are not readily usable as a precursor for the production of value-added materials and chemicals, thus yielding a technological gap in complete utilization of lignocellulosic biomass. Various processes have been developed and demonstrated for fractionation of technical lignins, with the purpose of increasing the suitability of technical lignins as a precursor for commercial production of fuels and chemicals. Fractionation of lignins reduces the amount of impurities and generates lignin streams with smaller internal variations in chemical and structural properties. Examples of such processes, including membrane filtration, solvent extraction, and acid gradient precipitation, are reviewed in this chapter.

**Keywords:** lignin, liquid–liquid extraction, membrane separation, biorefining
