About 20 years after the first discovery of planets orbiting other stars than the Sun, the big topic now in the field changed from "Are they there?" to "What are they like?". The recent years saw a tremendous progress in exoplanet characterization in terms of observing techniques and number of characterized planets, which allow now for comparative studies. I am going to review the current status of exoplanet characterization by transmission spectroscopy, discuss  the observational challenges and tell about my own contributions to the field.