Planetary Transits and Oscillations of stars (PLATO) is a space observatory under development by the European Space Agency for launch around 2024. PLATO will use a group of photometers to search for planet transits, to discover and characterize rocky extrasolar planets of all sizes around red dwarf stars, yellow dwarf stars like our Sun, and subgiant stars where water can exist in liquid state. A secondary objective is to study stellar oscillations to measure stellar masses and evolution.