Hot Jupiters offer a unique view into the nature of the planetary population in extra-solar systems due to their sizes, high temperatures and short orbits. A special class of these hot Jupiter-like planets - called Ultra-hot Jupiters - opens a window for broadening our understanding of atmospheric chemistry and thermal structure of the hottest extra-solar gas giants.  In my work I aim at understanding and interpreting high-resolution transmission spectra of ultra-hot Jupiter planets obtained by spectroscopic observations. The goal is to investigate the atmospheres of the hottest known exoplanets.