The solar cycle causes the Sun to change on timescales that can affect life. The maxima of cycles are marked by an increased occurrence of sunspots that cause solar flares and coronal mass ejections that can cause potentially harmful geomagnetic storms. Despite the fact that the existence of the solar cycle has been known for hundreds of years, we do not yet have an explanation as to why and how the solar cycle occurs, and we do not know enough to be able to predict the nature of future solar cycles.
Until quite recently, the only way to study the solar cycle was to look at surface indicators like the photospheric magnetic fields and sunspot numbers, however, with the availability of helioseismic data, we can now look inside the Sun to understand changes within. In this talk I shall describe some of the results that have been obtained with helioseismic data and show that solar cycle 24 has been very different from solar cycle 23, and discuss some of the implications of the results.