One of the main properties of young stellar objects (YSOs) is that they typically display brightness variability. This is true across all stages of young stellar evolution, and is observed over a wide range in wavelength (optical to sub-mm) and timescales (from a few hours to 100 yrs). This variability can be caused by various time-dependent phenomena associated with star formation. These include changes in accretion rates, geometric changes in the circumstellar disks, and hot/cold spots on stellar surfaces. In this talk I will give a review on the different classes of variability among YSOs and the physical mechanisms that lead to such variability. In here I will also mention my own contributions to the field as well as the connection of YSO variability and the future NASA SPHEREx mission.