  Galaxy cluster surveys are power tools for studying the dark energy. In
principle, the equation of state parameter w of the dark energy and its time
evolution can be extracted from large solid angle, high yield surveys that
deliver tens of thousands of clusters. Robust constraints require accurate
knowledge of the survey selection, and crude cluster redshift estimates must be
available. A simple survey observable like the cluster flux is connected to the
underlying cluster halo mass through a so--called mass--observable relation.
The calibration of this mass--observable relation and its redshift evolution is
a key challenge in extracting precise cosmological constraints. Cluster survey
self--calibration is a technique for meeting this challenge, and it can be
applied to large solid angle surveys. In essence, the cluster redshift
distribution, the cluster power spectrum, and a limited number of mass
measurements can be brought together to calibrate the survey and study the dark
energy simultaneously. Additional survey information like the shape of the mass
function and its evolution with redshift can then be used to test the
robustness of the dark energy constraints.
