Traditionally, the atmosphere of the Sun has been divided into four layers, starting with the photosphere at the bottom, followed by the chromosphere, the transition region, and the corona as the outermost region. The photosphere is a layer of only a few hundred kilometers thickness in which the temperature drops outwards from around 6000 K at the solar “surface” to around 4000 K at the “temperature minimum”. The photospheric gas changes from almost completely opaque at the bottom to almost completely transparent just a little more than one hundred kilometers higher up. Virtually all light which we receive from the Sun originates in the photosphere. Hence, most of the information we have about the Sun is derived from observations of this thin layer. Above the “temperature minimum”, the temperature appears to rise again, first gently – forming the chromospheric plateau – and then very steeply in the transition region at a height of about 2000 km above the solar “surface”. The chromosphere is thus defined as the zone between the “temperature minimum” and the transition region. It has obtained its name from the colorful appearance it exhibits during a total solar eclipse. Hydrogen is partially ionized throughout the chromosphere, which is neither in radiative equilibrium (RE) nor in local thermodynamic equilibrium (LTE). Increasingly detailed observations have revealed that the simple plane-parallel representation of the solar atmosphere outlined above is not tenable in any of its layers. In addition to being spatially inhomogeneous at almost all spatial scales, the solar atmosphere is also highly dynamic at almost all timescales. For recent reviews on the solar atmosphere see [i, 0, 3, 4]. Particularly useful textbooks on this subject are [h, k].