The study of planets orbiting other stars has become a mainstream branch of astronomy. Much of the focus is now on the discovery and characterization of exoplanets enough like the Earth that we can imagine life as we know it could be comfortable there: Goldilocks planets with solid surfaces at the right temperature for water to be liquid, enveloped by secondary atmospheres with molecules suitable for the invention of life. Characterizing the structure and composition of such planets depends on knowledge of the bulk density, and interpretation of their atmospheres relies on knowledge of the surface gravity and its role in setting the scale height. Thus transiting planets are playing a key role, because they are the main source of exoplanets for which both radius and mass can be determined. Progress in this field has been driven by improved technologies. Space missions have supported major breakthroughs, but ground-based determinations of planetary masses are equally important. We are already using remote sensing to study the atmospheres of gas giant planets, and the dream is to use spectroscopy of the atmospheres of rocky planets to detect signs of pollution by life.