Measurements of activity indicators including stellar X-ray and UV emission and wind flux are essential for calculating the photochemistry and hydrodynamical outflow rate in exoplanet atmospheres at the present time, and it is also important to estimate what the stellar fluxes were at earlier times and will likely become in the future. Assessing the habitability of an exoplanet requires knowledge of the host star’s emission back to the protoplanetary stage. We therefore need to know how emission depends on stellar properties including mass, age, and rotation rate, which depends on a star’s age and mass. This chapter reviews the systematic studies of stars in different mass ranges that have led to correlations and scaling laws for the dependence of stellar emission on age and rotation rate.