Atmospheres of exoplanets are our only window into the physical and chemical processes occurring in these distant worlds. These processes are important tracers of the origins and evolution of planetary systems, including our Solar System. In this broader context, we could better understand how common (or unique) are the conditions leading to the emergence of life, which could leave its spectroscopic imprints precisely into planetary atmospheres. This lecture is focused on one of the available techniques to study exoplanetary atmospheres: transit transmission spectroscopy. After describing some basics about this technique, I will illustrate through some case studies how it can practically bring observational constraints on these remote and exotic atmospheres.