  The dynamic activity of stars such as the Sun influences (exo)planetary space
environments through modulation of stellar radiation, plasma wind, particle and
magnetic fluxes. Energetic stellar phenomena such as flares and coronal mass
ejections act as transient perturbations giving rise to hazardous space
weather. Magnetic fields -- the primary driver of stellar activity -- are
created via a magnetohydrodynamic dynamo mechanism within stellar convection
zones. The dynamo mechanism in our host star -- the Sun -- is manifest in the
cyclic appearance of magnetized sunspots on the solar surface. While sunspots
have been directly observed for over four centuries, and theories of the origin
of solar-stellar magnetism have been explored for over half a century, the
inability to converge on the exact mechanism(s) governing cycle to cycle
fluctuations and inconsistent predictions for the strength of future sunspot
cycles have been challenges for models of solar cycle forecasts. This review
discusses observational constraints on the solar magnetic cycle with a focus on
those relevant for cycle forecasting, elucidates recent physical insights which
aid in understanding solar cycle variability, and presents advances in solar
cycle predictions achieved via data-driven, physics-based models. The most
successful prediction approaches support the Babcock-Leighton solar dynamo
mechanism as the primary driver of solar cycle variability and reinforces the
flux transport paradigm as a useful tool for modelling solar-stellar magnetism.
