Sex-steroid hormones have widespread effects on the human brain and are involved in numerous physiological processes throughout life. In addition, they have a strong influence on cognition and emotion. Transgender persons undergoing cross-sex hormonal treatment offer the unique opportunity to study these effects in vivo. Here, we utilized a multimodal, MR-based neuroimaging approach using (1) structural MR for voxel-based morphometry (VBM) and cortical thickness assessment, (2) diffusion-weighted imaging (DWI) for diffusion changes and (3) resting-state fMRI for functional connectivity metrics with 3 and 7 Tesla (ultra-)high field MR to assess structural changes (gray matter and white matter) and alterations in functional connectivity due to hormonal treatment. In addition, a methodological comparison between structural 3 T and 7 T VBM data was conducted in order to validate these measurements at ultra-high fields. The methodological investigation of VBM data yielded differences between the magnetic field strengths with better test-retest reproducibility for 3 T data. This was likely due to signal intensity variations at 7 T with the sequences we used. Although 7 T assessments deliver several benefits including higher resolution, our results motivated us to use the 3 T data for the subsequent structural analysis. Nevertheless, improvement of sequences and adapted software for higher fields will help to get rid of the most common artefacts. Gray matter volume (GMV) analysis after 4-months hormonal treatment predominantly revealed changes for Male-to-Females (MtF) after anti-androgen and estradiol treatment. Significant volumetric decreases in the hippocampus were observed, while the volume of the ventricles increased. Furthermore, progesterone levels were associated with these volumetric changes. The white matter assessments using diffusion tensor imaging (DTI) were carried out 4 weeks and 4 months following the baseline scan. Opposing effects of both treatment regimens were found. In the Female-to-Male (FtM) cohort, increased fractional anisotropy (FA) values and reduced mean diffusivity (MD) were observed, while in the MtF group, the opposite effects were found. In addition, hormonal fluctuations correlated with MD and FA parameters. Furthermore, we combined gray matter (GM) morphometry and white matter (WM) structural connectivity measurements in a multimodal fashion. FtMs were analyzed before testosterone treatment and after one month of continuous administration. The VBM analysis revealed reduced GMV with increasing levels of testosterone in Brocas as well as in the Wernickes area. Probabilistic tractography in the path of the extreme capsule, connecting both areas above, indicated a negative association between testosterone and MD values. This could be interpreted as a strengthening of this fiber tract. Finally, resting-state functional connectivity showed increased connectivity metrics between these two brain areas, correlated with increasing levels of testosterone. In this thesis we could demonstrate that sex-steroid hormones have widespread effects on the living human brain by altering GM, WM and functional connectivity metrics in transgender persons.