<p>Developmental immunotoxicology (DIT) is emerging as a critical area in regulatory toxicology, driven by the recognition that the developing immune system is particularly vulnerable to xenobiotic exposure. Disruptions occurring during fetal or early postnatal life may result in long-lasting alterations in immune competence, tolerance, and disease susceptibility. This review provides a comprehensive overview of immune system development, highlighting key developmental stages from embryogenesis to postnatal maturation and identifying windows of heightened immune system sensitivity to toxicants. By integrating mechanistic insights and methodological advances, this review aims to support the improvement and extension of DIT testing frameworks and the development of predictive tools for regulatory and research applications. Recent advances in New Approach Methodologies offer promising alternatives for modeling human immune ontogeny, while highlighting the challenge of ensuring adequate coverage of critical developmental mechanisms and windows of susceptibility relevant to immunotoxicity. The integration of physiological maps and multi-omics technologies enhances mechanistic understanding, while epidemiological associations between exposures and functional endpoints underscore the real-world relevance of DIT and can identify biomarkers to guide the further development of relevant and sensitive models. Despite these advances, challenges remain, including the scarcity of human reference data, the lack of standardized protocols, and the need for validated test batteries covering diverse mechanisms once the tests have been refined. Addressing these gaps is essential to support the regulatory uptake of DIT data and to advance predictive, mechanistically anchored, and ethically sound strategies for DIT testing.</p>