The abundance of massive clusters of galaxies is a sensitive probe of how cosmic structures have grown throughout the history of the universe. This can provide constraints on cosmological parameters that are complementary to the constraints obtained e.g. from the cosmic microwave background or from high-redshift supernovae.
Any effort to do precision cosmology with clusters requires careful calibration of the scaling relations between various observables of the baryonic component of the cluster and the total cluster mass, which is dominated by non-baryonic dark matter.
I will present results using weak gravitational lensing to determine these relations and provide new constraints on the comological parameters governing structure growth in the universe. Finally, I will discuss prospects for using high-redshift galaxy clusters (detected e.g. with the Planck satellite) to constrain dark energy.