  Quasi-periodic pulsations (or QPPs) are periodic intensity variations in the
flare emission, across all wavelength bands. In this paper, we review the
observational and modelling achievements since the previous review on this
topic by Nakariakov & Melnikov (2009). In recent years, it has become clear
that QPPs are an inherent feature of solar flares, because almost all flares
exhibit QPPs. Moreover, it is now firmly established that QPPs often show
multiple periods. We also review possible mechanisms for generating QPPs. Up to
now, it has not been possible to conclusively identify the triggering mechanism
or cause of QPPs. The lack of this identification currently hampers possible
seismological inferences of flare plasma parameters. QPPs in stellar flares
have been detected for a long time, and the high quality data of the Kepler
mission allows to study the QPP more systematically. However, it has not been
conclusively shown whether the time scales of stellar QPPs are different or the
same as those in solar flares.
