Groundwater contamination by halogenated organic compounds, especially chlorinated and fluorinated ones, threatens freshwater sources globally. Nanoscale zero-valent iron (NZVI) has been extensively studied (>5000 publications) and deployed for in situ groundwater remediation, but NZVI selectivity for contaminants is poor, reactive lifetimes are short, and it cannot promote defluorination reactions. Recently, sulfidized NZVI (SNZVI) has emerged, and has revitalized academic and industrial interests in this material for remediation. Sulfidation broadens the range of reactive contaminants, and significantly increases the selectivity and reactive lifetime of NZVI by 2 orders of magnitude, while inhibiting the undesirable H2 evolution reaction between Fe0 and water. This Account provides a state-of-the-art understanding of the chemical properties controlling the reactivity and selectivity of SNZVI and will advance the field toward the rational design of efficient groundwater remediation materials. SNZVI is a complex mixture of reactive body-centered cubic (BCC) metallic iron and unspecified iron sulfides. Most published SNZVI research has aimed at exploring the breadth of its reactivity toward various environmental contaminants rather than understanding the factors that influence the reactivity of this complex mixture of materials. Recent works from our laboratory have aimed at tuning the synthesis conditions to control the amount and speciation of sulfur in the SNZVI structure, and elucidating how these structural changes result in physicochemical properties (e.g., hydrophobicity, electron-transfer resistance, and H adsorption sites) that provide desirable reactivity and selectivity for important groundwater contaminants. This Account explains the reasons for the more desirable properties of SNZVI compared to NZVI. The degradation pathways, and reactive sites (Fe or S sites) and species (direct electron transfer or atomic H) of SNZVI for the dechlorination of trichloroethene and defluorination of florfenicol are determined from batch experiments, theoretical calculations, and analysis of degradation products. A better understanding of why SNZVI is reactive with C–F bonds under ambient conditions may also promote the use of SNZVI and its derivatives for the defluorination of emerging groundwater contaminants. Finally, this Account provides guidance for measuring and reporting the complex material properties of SNZVI. This will enable comparisons between future studies to elucidate the reasons for differences in the reactivity of SNZVI synthesized by different research groups. Overall, this Account unveils the structure–property–performance relationships of SNZVI, makes strides toward the controlled synthesis and rational design of robust SNZVI with properties tailored for specific application scenarios, and provides mechanistic insights into SNZVI materials for in situ groundwater remediation of chlorinated and fluorinated contaminants.