  This topical collection summarizes recent advances in observing and modeling
irradiance variations of the Sun and Sun-like stars, emphasizing the links
between surface magnetic fields and the resulting solar and stellar
variability. In particular, the articles composing this collection summarize
recent progress in i) solar-irradiance measurements; ii) modeling of solar- and
stellar-irradiance variability; and iii) understanding of the effects of such
variability on Earth's climate and exoplanet environments. This
topical-collection overview article gives background and more details on these
aspects of variability.
