Some injuries to the developing nervous system can be detected with traditional evaluation for morphologic pathology, but many early injuries differ in character from those that are produced later in life. Such injuries arise from interference with developmental processes, rather than destruction of tissue. For example, an injury which kills neurons in the mature CNS leads to gliosis and a reduction in neuronal density, but a reduction in the number of neurons produced during development is not likely to lead to gliosis, and typically affects tissue volume rather than cell density. Some effects of developmental insults, such as misplaced and misoriented neurons, are never seen after adult injury. Functional effects reflect the role of the CNS in physiological regulation as well as in behavior. To evaluate CNS for developmental injury, it is necessary to know something about the structural and functional outcomes already recognized to result from teratogens and how these effects are related to time of exposure and time of testing.