Clusters of galaxies are powerful and in many ways unique probes of cosmology. A new generation of large X-ray, optical, and Sunyaev-Zel'dovich Effect (SZ) cluster surveys (SDSS, SPT, ACT, DES) are producing or will produce cluster samples of tens of thousands of objects. These datasets have the statistical power to constrain cosmological parameters with unprecedented precision. In 2006, we expect several significant results from the first large cluster surveys, such as the 400 square degree X-ray survey, the SDSS (optical and weak lensing), and initial results from the first blind SZ surveys (SZA, AMI, and APEX-SZ). At the same time, we have witnessed many theoretical advances in understanding the properties of galaxy clusters and effects of galaxy formation and AGN feedback are being actively explored using a new generation of high-resolution cosmological simulations. The goal of the workshop is to define strategies for the use of clusters as powerful cosmological probes in ways which take full advantage of the statistical power of the observational samples.