Now that the existence of exoplanets and their abundance in our Galaxy is an established knowledge, our attention is focusing on their characterization. Transmission spectroscopy is the best technique to constrain the atmospheric properties of transiting exoplanets: during a transit, the light blocked from the atmosphere of the planet leaves an imprint in the light coming from the star. We are able to discover many atmospheric details joining multi-colour photometry and spectroscopy, both ground and space-based. It’s also possible to study non-transiting exoplanets atmospheres, catching photons coming directly from the planet. With the growing sample of known exoplanets, and filling the gaps caused by detection biases with new generation instruments, we are starting building an ensemble view of their physical characteristics.