Most of the information about stars is brought by photons that originate in stellar atmospheres. Consequently, understanding and modelling the conditions in the atmosphere of a star is of crucial importance for understanding stellar observations and the star as a whole. While there are a lot of similarities between studies of solar and stellar atmospheres, there is also a key difference: solar studies largely benefit from the ability to resolve features on the solar surface. In particular, the Sun provides a unique opportunity to directly study the emergence and disappearance of magnetic fields in its atmosphere as well as the sophisticated interaction between these fields and plasma. Furthermore, high-resolution solar observations revealed that besides a strong vertical stratification with depth, the solar atmosphere has a very rich and complicated horizontal structure. Progress in solar observations brought about several recent and exciting results which are of direct relevance for understanding stellar atmospheres.

Understanding and modeling of solar-irradiance variability is important not only for solar physics but also for solar-terrestrial and solar-stellar studies. The latest irradiance measurements call into question aspects of currently-available empirical and semi-empirical models of solar-irradiance variability. A new generation of significantly more realistic physics-based irradiance models can now be created to incorporate recent advances in modeling and observing the solar atmosphere. This next generation of irradiance models will include new advances in MHD, surface flux transport, and radiative transfer simulations as well as new state-of-the-art solar data. By relying on physics-based understandings rather than merely empirical relationships established for the Sun, these new models will also allow more direct and physical extrapolations to other stars, opening a new regime for solar-stellar connection studies, as well as improved long-term estimates of historical solar variability.

The unprecedented precision of stellar brightness measurements achieved by the planet-hunting space telescopes initiated a new era in stellar photometric variability investigations. Understanding stellar brightness variations is of great interest to the solar, stellar, and exoplanetary communities, for the following reasons: Stellar brightness variations can provide constraints on the historical solar variability and solar role in climate change, as well as they allow to determine stellar magnetic cycles' properties. Moreover, stellar brightness variations are a limiting factor for detection and characterisation of the exoplanets via transit photometry. Recently, a plethora of observational data have pushed ahead theoretical studies aiming at developing methods for extracting information about stars and their planets from the available records of brightness variations. These studies can greatly profit from knowledge acquired by studying the Sun. Thus the way forward is to focus on the solar-stellar comparison and examining how the solar paradigm can help us to explain variability of other stars and develop criteria for distinguishing between typical photometric signatures of intrinsic stellar variations and exoplanet transits.