Galaxy clusters are the most massive virialised structures in the
Universe. Their number density depends on both the geometry of the
Universe and the rate at which structure grows.
Galaxy clusters are also rich in relatively dust-free, early-type
galaxies, which are known to host only Type Ia supernovae. Evidence from
several surveys now suggest that Type Ia supernovae hosted by early-type galaxies
are better standard candles than Type Ia supernovae hosted by galaxies of
other types. In this talk I will describe how clusters can be used as a dark energy
probe, both through the number density of clusters and the through the
Type Ia supernovae that can be found in them.