Helioseismology uses the Sun’s natural oscillations to probe beneath its visible surface. The dominant solar oscillations are sound waves, also known as p modes as the main restoring force is a pressure differential. By observing the properties of these p modes and comparing to models, we can build up profiles of the solar interior. Interestingly, though, many of the properties of p modes are not constant in time. Instead they vary through the Sun’s 11yr magnetic activity cycle, indicating that the p modes are influenced by the Sun’s internal magnetic field. In this talk, I will discuss these variations in detail, describing what we can and can’t yet use these variations to reveal about the solar cycle. The Sun, however, is not the only star in which we can observe p modes and not the only star where the properties of these p modes are observed to vary on activity cycle time scales. I will therefore finish this talk by expanding out to other stars, discussing the potential of asteroseismology to reveal stellar signatures of magnetic activity, not just stars like our Sun, but also on more evolved stars as well.