  The 11-year sunspot cycle has many irregularities, the most promi- nent
amongst them being the grand minima when sunspots may not be seen for several
cycles. After summarizing the relevant observational data about the
irregularities, we introduce the flux transport dynamo model, the currently
most successful theoretical model for explaining the 11-year sunspot cycle.
Then we analyze the respective roles of nonlinearities and random fluctuations
in creating the irregularities. We also discuss how it has recently been
realized that the fluctuations in meridional circula- tion also can be a source
of irregularities. We end by pointing out that fluctuations in the poloidal
field generation and fluctuations in meridional circulation together can
explain the occurrences of grand minima.
