The co-precipitation of Fe(III) (oxyhydr)oxides with arsenic (As) is one of the most widespread approaches to treat As-contaminated groundwater in both low- and high-income settings. Fe-based co-precipitation of As occurs in a variety of conventional and decentralized treatment schemes, including aeration and sand filtration, ferric chloride addition and technologies based on controlled corrosion of Fe(0) (i.e., electrocoagulation). Despite its ease of deployment, Fe-based co-precipitation of As entails a complex series of chemical reactions that often occur simultaneously, including electron-transfer reactions, mineral nucleation, crystal growth, and As sorption. In recent years, the growing use of sophisticated synchrotron-based characterization techniques in water treatment research has generated new detailed and mechanistic insights into the reactions that govern As removal efficiency. The purpose of this critical review is to synthesize the current understanding of the molecular-scale reaction pathways of As co-precipitation with Fe(III), where the source of Fe(III) can be ferric chloride solutions or oxidized Fe(II) sourced from natural Fe(II) in groundwater, ferrous salts or controlled Fe(0) corrosion. We draw primarily on the mechanistic knowledge gained from spectroscopic and nano-scale investigations. We begin by describing the least complex reactions relevant in these conditions (Fe(II) oxidation, Fe(III) polymerization, As sorption in single-solute systems) and build to multi-solute systems containing common groundwater ions that can alter the pathways of As uptake during Fe(III) co-precipitation (Ca, Mg bivalent cations; P, Si oxyanions). We conclude the review by providing a perspective on critical knowledge gaps remaining in this field and new research directions that can further improve the understanding of As removal via Fe(III) co-precipitation.