One of the major uncertainties that can affect a structure is the load. All of the loads that affect the structure during its lifetime are variable parameters, even the dead load as will be explained in this chapter. However, the live load is variable with time, and there are many studies and tests that have been applied to define the variability on the live load. Therefore, this chapter will try to present all of the important research studies, with different approaches, to define the phenomena of the live load and its characteristics.