Variability pervades the cosmos at all time scales: particularly in X-rays many kinds of sources display a variety of temporal behaviors. XMM-Newton, with its combination of high performance and over 15 years of public data, is a goldmine for exploring the variable X-ray sky. EXTraS is a EU- funded project aiming to characterize as carefully as possible the variability of almost all XMM sources. I will present the various aspects of X-ray variability that we are exploring in this project. I will describe some of the techniques we developed for dealing with a variable background and low- counts statistics. I will overview some early results, like flares from a young stellar object, the first pulsar detected in the Andromeda galaxy, and two accreting pulsars powering ultraluminous X-ray sources. Finally, the products of EXTraS, including a catalogue and software tools, will be released to the community, enabling you to produce your own science from X-ray variability