evidence: Introduction to the Solar Activity Cycle: Overview of Causes and Consequences [...] The solar activity cycle, a near-periodic variation generally close to a period of 11 years, appears prominently in most parameters that describe solar phenomena. Records of variations, originally based on sunspot counts, show that this has been the case for historic times, even if during certain intervals lasting decades and longer sunspots were rare. This demonstrates the existence of a timing engine in the Sun that appears to control or influence all aspects of solar phenomena and also beyond, including phenomena elsewhere in the solar system that are affected by the Sun. In recent decades, understanding the solar activity cycle has progressed from being of purely scientific interest to a practical need because of the recognition that solar phenomena affect the Earth’s space environment in ways that can be potentially damaging. It is generally agreed that the timing engine in the Sun causes a nearly periodic generation and evolution of magnetic fields in the solar interior and on the solar surface, more explicitly, that a dynamo mechanism generates the Sun’s magnetic field. [...] However, the very complex variations in the spectrum of the solar irradiation as a function of the activity cycle, in particular as a function of the strength, distribution and short- and medium-term variability of the magnetic features remain to be explained (Yeo et al. 2014, this issue). Progress will be made by more accurate measurements, in particular spectral measurements, combined with more detailed models of the spectral irradiance.