Polymer flooding is a chemical enhanced oil recovery method that improves waterflood performance by reducing the mobility of the injected aqueous phase and enhancing reservoir sweep. This review comprehensively examines the principles and mechanisms of polymer flooding, the polymer systems used, their interactions with reservoir conditions, and the operational factors that influence performance. It also evaluates laboratory and numerical methods, field applications, economic and environmental considerations, and recent technological advances. The discussion covers conventional synthetic polymers, particularly partially hydrolysed polyacrylamide, as well as biopolymers, associative polymers, thermoviscosifying polymer (TVP) systems, zwitterionic and salt-tolerant polymers, and polymeric nanocomposites. Particular emphasis is placed on adsorption and retention, inaccessible pore volume, mechanical degradation, thermal and chemical stability, injectivity, reservoir heterogeneity, and polymer–rock interactions. Laboratory core-flooding experiments and numerical simulations are examined as key methods for polymer evaluation and field-scale design. Case studies from China, Canada, Oman, and other regions demonstrate that performance is strongly dependent on reservoir-specific conditions. The review also considers economic and environmental factors, including chemical use, water management, produced-water treatment, and the potential sustainability advantages of advanced biopolymer systems. Recent developments in smart polymers, nanocomposites, hybrid injection strategies, and data-driven reservoir optimisation are also assessed. Overall, reliable and economically viable polymer flooding requires an integrated approach that brings together polymer chemistry, reservoir characterisation, laboratory testing, numerical modelling, and field surveillance.