Nanoscale zerovalent iron (nZVI) is increasingly utilized for the removal of halogenated organic contaminants, which pose significant global environmental and human health concerns. This review summarizes the recent theoretical studies on mechanisms underlying the remediation of halogenated organic contaminants in groundwater using nZVI and its modifications, including sulfidated nZVI and bimetallic nZVI. By emphasizing the effectiveness of these materials in degrading and removing halogenated organic pollutants, we highlight the crucial role of the high reactivity of higher-index Miller stepped surfaces. These stepped surfaces can better represent the characteristics and reactivity of nZVI and often serve as active sites for contaminant removal. However, such active sites are susceptible to water oxidation. Therefore, modifications with sulfur or metal dopants can enhance its stability and reactivity. Recent theoretical studies reveal that dopants significantly influence nZVI performance in aqueous environments. State-of-the-art theoretical computations thus provide essential insights into optimizing nZVI for the efficient degradation of halogenated organic contaminants.