Fe(0) electrocoagulation (Fe-EC) is a promising treatment technology for water and wastewater owing to its simplicity, cost-effectiveness, and environmental sustainability. Throughout the extensive development history of the Fe-EC process, research has been predominantly focused on pollutant adsorption and coprecipitation driven by the hydrolysis products of ferric ions. However, recent advancements have highlighted the generation of redox-sensitive Fe(II) species that include aqueous, complexed, structural, and adsorbed Fe(II) for redox transformation of contaminants within the Fe-EC process. In this critical review, we elucidate the formation conditions and redox properties of the various Fe(II) species, and underscore the pivotal roles these species play in facilitating the transformation of contaminants. Specially, the sequestration and reduction of contaminants by structural and adsorbed Fe(II) species constitute a fascinating but less-traveled area of study, given their rapid kinetic rates and the potential for resource recycling. In summary, this review delves into the redox transformations of pollutants within the Fe-EC process, provides guidelines for the application of diverse Fe(II) species, and offers a forward-looking perspective on the future development of this well-established technology.