It has long been known that the developing immune system is more sensitive and susceptible than the adult immune system to some drugs and environmental contaminants. However, notable advances have been made in the database of studies supporting developmental immunotoxicity (DIT) over the past 5 years. There is considerable evidence that responses of the immune system can be quantitatively or qualitatively different from normal adult responses when xenobiotic exposure occurs during critical periods of immune system development. Qualitative differences of DIT relative to adult exposures include examples of more persistent effects, a latency of effects, and immune dysfunction that is fundamentally different than effects observed when adults are exposed. A symposium was presented at the Society of Toxicology annual meeting to provide an update on advances in the maturing field of developmental immunotoxicology and to facilitate discussion on the range of DIT and later life effects following developmental exposure. In particular, presentations focused on implications of neuroendocrine cross-talk for DIT, the association between developmental air pollutant exposure and asthma, and recent evidence that developmental exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin may increase the risk of autoimmune responses. Several important concepts relative to DIT assessment were illustrated, i.e., (1) Screening for immunosuppression alone is not sufficient to identify all potential immunotoxic effects; (2) DIT cannot be reliably predicted from studies that only utilize adult exposures; (3) Functional testing protocols are preferred in the assessment of DIT; (4) Gender-related differences should be routinely assessed; (5) Latency (i.e., later-life adverse outcomes resulting from developmental exposures) is an important consideration that cannot be detected in adult exposure studies; and, (6) There is increasing support for DIT testing protocols with continuous exposure throughout development until the immune assay is performed.