  A review of solar cycle prediction methods and their performance is given,
including forecasts for cycle 24 and focusing on aspects of the solar cycle
prediction problem that have a bearing on dynamo theory. The scope of the
review is further restricted to the issue of predicting the amplitude (and
optionally the epoch) of an upcoming solar maximum no later than right after
the start of the given cycle. Prediction methods form three main groups.
Precursor methods rely on the value of some measure of solar activity or
magnetism at a specified time to predict the amplitude of the following solar
maximum. Their implicit assumption is that each numbered solar cycle is a
consistent unit in itself, while solar activity seems to consist of a series of
much less tightly intercorrelated individual cycles. Extrapolation methods, in
contrast, are based on the premise that the physical process giving rise to the
sunspot number record is statistically homogeneous, i.e., the mathematical
regularities underlying its variations are the same at any point of time, and
therefore it lends itself to analysis and forecasting by time series methods.
Finally, instead of an analysis of observational data alone, model based
predictions use physically (more or less) consistent dynamo models in their
attempts to predict solar activity. In their overall performance precursor
methods have clearly been superior to extrapolation methods. Nevertheless, some
extrapolation methods may still be worth further study. Model based forecasts
have not yet have had a chance to prove their skills. One method that has
yielded predictions consistently in the right range during the past few solar
cycles is that of K. Schatten et al., whose approach is mainly based on the
polar field precursor. The incipient cycle 24 will probably mark the end of the
Modern Maximum, with the Sun switching to a state of less strong activity.
