Polyacrylonitrile fibers feature linear macromolecules with a polyacrylonitrile mass ratio of at least 85 percent. Modacrylic fibers feature linear molecules with an acrylonitrile mass ratio of at least 50 percent and less than 85 percent in the chain. An overview of polyacrylic fibers addresses DIN ISO 2076 definitions; fiber invention and development; raw materials, including acrylonitrile synthesis, polymerization, and copolymerization; spinning solutions; wet, dry, gel, and melt extrusion processes; aftertreatments, including drawing, washing, spin finishing, drying, crimping, relaxing, converting, packing, and dyeing; solvent recovery; wet extrusion, dry extrusion, and continuous dry extrusion lines; makeup forms, including staple fibers, tow, and filament yarns; fiber properties; modified fiber properties, including bicomponent, spun dyed, shrinkable, absorbent, and modacrylic fibers and asbestos substitutes; applications of polyacrylonitrile staple fibers, filament yarns, and modacrylic staple fibers; recycling; and production data. 146 refs.