What can we learn about young stellar objects from their variability?

Variability is practically a defining property of young stellar objects. In this talk I will summarize variability statistics during pre-main sequence evolution, focusing on low and intermediate mass stars. I will present different variability patterns and their physical reasons. I will highlight three special topics: regular accretion variability of T Tauri stars, extreme accretion outbursts of young eruptive stars, and pulsed accretion observed in some young binaries. I will discuss how variability, if accompanied by significant luminosity changes in the system, may substantially alter the geometrical, mineralogical, or chemical structure of the disk, and may consequently affect the disk’s planet-forming capabilities. Finally, I will show some prospective future directions in this research field, using data from on-going or upcoming surveys.