Brain-resident immune cells are involved in the protection against microbes invading the brain. In addition, their essential role in regulating physiological processes, including neurogenesis and neuronal migration, has been documented. In this chapter, we will provide a general introduction to immune cell types that reside and infiltrate the brain. Furthermore, the impact of brain-resident immune cells on brain physiology will be discussed. The phenotype of immune cells is known to be impaired in neurodevelopmental disorders. Thus, we will describe how the accumulation of dysfunctional immune cell types contributes to the pathogenesis of this group of diseases, with a particular emphasis on the connection between immune dysfunction, inflammation, and oxidative stress. Finally, possible interventions to counteract neurodevelopmental disorders by targeting oxidative stress and inflammation will be summarized.