The measurement and interpretation of oscillations or pulsations of stellar photospheres. It is the extension of helioseismology to other stars. Asteroseismology is often used to refer to the study of weak, solar-like oscillations in stars, but can also include the larger amplitude variations of, for example, rapidly oscillating Ap stars and Beta Cephei stars. Solar-like oscillations have been identified in several dwarf and subgiant stars, with over 30 oscillation modes detected in Alpha Centauri A. In most cases the disk of the star cannot be resolved so it is global oscillations that are measured, which probe the deeper layers of the star. Two techniques are used to measure stellar oscillations: photometry and Doppler spectroscopy. The former is difficult for detecting solar-like oscillations as the brightness variations are typically a few millionths of a magnitude, at the limit of what is measurable from ground-based observatories. The perfect seeing available in space has, however, allowed such tiny oscillations to be measured by satellites such as Microvariability and Oscillations of Stars (MOST), CoRoT, and Kepler. Solar-like oscillations have been detected at ground-based observatories using radial-velocity spectrometers that measure the Doppler motions on the stars’ surfaces. Observations are made at several sites around the globe to provide continuous coverage for long periods.