Countercurrent extraction of lignocellulosic biomass such as trees, grasses, shrubs, and agricultural residues or waste involves the separation of cellulose fibers from other constituents, for subsequent use in the manufacture of paper, plastics, ethanol, and other industrial chemicals. Systems and methods involve continuous, multiple processing steps that may include chemical reactions with mixing at elevated temperature and/or pressure, efficient reagent or solvent utilization, filtration at elevated temperature and/or pressure, controlled discharge of liquid and solid products, and energy recuperation.