  The total and spectral irradiance varies over short time scales, i.e. from
days to months, and longer time scales from years to decades, centuries, and
beyond. In this talk we review the current understanding of irradiance changes
from days to decades. We present the current status of observations and discuss
proposed reconstruction approaches to understand these variations. The main
question that ultimately needs to be answered is what are the physical
processes that could explain the enhanced heating of the photosphere,
chromosphere, transition region, and corona, leading to a change in the solar
radiative output at various wavelengths. As semi-empirical models allow us to
reproduce the solar spectrum over a broad wavelength range, they offer a
powerful tool to determine the energy necessary to heat certain layers and at
the same time balance the radiative losses.
