  A small amount of polymer additives can cause substantial reduction in the
energy dissipation and friction loss of turbulent flow. The problem of
polymer-induced drag reduction has attracted continuous attention over the
seven decades since its discovery. However, changes in research paradigm and
perspectives have triggered a wave of new advancements in the past decade. This
review attempts to bring researchers of all levels, from beginners to experts,
to the forefront of this area. It starts with a comprehensive coverage of
fundamental knowledge and classical findings and theories. It then highlights
several recent developments that bring fresh insights into long-standing
problems. Open questions and ongoing debates are also discussed.
