The enzyme aromatase coded by the gene CYP19A1/Cyp19a1 (humans/animals) catalyses the conversion of androgens into oestrogens, which play an important role in brain function from development through adulthood. Together with the sex hormones, aromatase seems to influence behaviour and cognition by affecting neurogenesis, differentiation, neuroplasticity, and neuroprotection. It is highly expressed in the limbic brain, suggesting a link to sex differences in mental health, including nicotine addiction, gambling disorder and resilience to early life stress (ELS) as well as structural properties of the brain. However, the underlying molecular mechanisms remain unclear. The studies of the present dissertation explored the distribution and influence of aromatase and sex hormones in rat and human brains, using different molecular biological methods, cognitive testing, as well as multimodal neuroimaging. Using fluorescence in-situ hybridization in the rat brain, Cyp19a1 expression was observed in the limbic regions and found to be higher in males compared to females, with the highest expression in the medial amygdala and the bed nucleus of the stria terminalis. Most Cyp19a1-expressing cells were GABAergic, some glutamatergic, and a small population of astrocytes expressing the gene was found. Furthermore, the expression of Cyp19a1 was observed to be lower in the medial prefrontal cortex (mPFC) of male rats exposed to ELS compared to controls, and a relationship between ELS and DNA methylation in the Cyp19a1 gene was detected. Using magnetic resonance imaging combined with [¹¹C]cetrozole positron emission tomography in young adult women, aromatase availability (AA) was observed in the thalamus, the amygdala and the hypothalamus. A positive correlation with grey matter volume in those regions was found, together with a relationship between AA and the volume and cortical thickness of the mPFC and the volume and microstructure of the fornix. Additionally, an acute dose of nicotine was able to reduce AA in the thalamus. Experimental testing in young women showed an effect of menstrual cycle phase on reward-based decision-making and suggested a possible interaction between oestradiol and nicotine. In conclusion, these findings show for the first time in rats that aromatase is sex-, region- and cell type-specifically expressed and is affected by ELS. Moreover, in women aromatase is shown to be related to brain morphology and can be inhibited by nicotine, while fluctuating oestradiol influences cognition both alone and possibly in an interplay with nicotine. These findings advance our knowledge on the role of aromatase in the brain and the theoretical framework of psychoneuroendocrinology.