The development of redox-active polymers (RAPs) as organic electrode materials is foreseen as a promising solution for safe, sustainable and high-performance electrochemical energy storage technologies. Among the different families of RAPs, poly(catechol)s are especially interesting due to their remarkable degree of physicochemical and electrochemical versatility. This chapter describes their intriguing and unique properties and details the different routes towards poly(catechol)s, including bioresources (e.g., lignins, tannins, eumelanins, etc.) and other advanced synthetic strategies. Moreover, their application as active materials in different types of electrochemical energy-storage systems such as capacitors, rechargeable static batteries and redox flow batteries is also discussed. Poly(catechol)s were first applied as pseudocapacitive electrodes, either combined with carbon additives or with intrinsically conducting polymers, in hybrid capacitors. Lately, poly(catechol)s were also investigated as electrode materials, both cathode and anode, in numerous rechargeable static batteries. As anode materials, they were first used to obtain activated carbons or, in a more interesting approach, designed to undergo superlithiation (or sodiation) reactions in alkali metal-ion batteries. Taking advantage of the reversible redox couples involving catecholate/ortho-quinone transformations and their propensity for the metal-ion coordination/uncoordination reactions, poly(catechol)s have been also recently proposed as “universal” cathodes in monovalent Li-ion, Na-ion, multivalent Mg-ion, Zn-ion and all-polymer batteries. The use of lignin as an abundant and extremely low-cost biopolymer for sustainable redox flow battery is also discussed here.