Vitamin D belongs to the super family of nuclear steroid transcription regulators that include thyroid hormones, vitamin A, androgens and the glucocorticoids, agents that have all been closely linked with how the brain develops. Nevertheless, neuroscientists have been slow to direct their attention to vitamin D with some authors referring to it as the “neglected” neurosteroid. Our group has produced a number of large epidemiological studies showing maternal vitamin D deficiency increases the risk of either schizophrenia or autism in offspring. These disorders are believed to have a developmental origin. To understand the neurobiology behind this association, our group using either our animal model of maternal vitamin D deficiency or the active hormone, 1,25dihydroxyvitamin D, have pioneered studies that show a) vitamin D catabolism and signaling occurs in the brain b) vitamin D affects how neurons migrate, divide and differentiate; c) vitamin D directly and indirectly regulates the expression of important neurotrophic factors, neurotransmitters and factors involved in neurotransmitter turnover; d) Vitamin D deficiency affects developing and adult brain structures; e) Maternal Vitamin D deficiency produces long-term behavioural changes f) Maternal 1,25dihydroxyvitamin D treatment rescues behavioural deficits induced by other adverse in utero exposures. Our basic work in cell systems and animal models has firmly established the neurobiology for how vitamin D deficiency in utero could produce later psychiatric disorders. Our findings have converged on dysfunction of the neurotransmitter dopamine. Dopamine abnormalities remain a cornerstone of many psychiatric disorders. Increasingly, we have turned our attention to the molecular processes behind how vitamin D regulates dopamine synthesis/turnover, terminal connectivity and release in developing neurons and in adult brains. For the past 25 years our work has consistently shown Vitamin D levels have important consequences for how a brain is built and functions. In short, it is a developmental neurosteroid. No longer should this neurosteroid be “neglected.”