Using a probabilistic photometric redshift-based approach combining Spitzer mid-IR and NOAO optical data, we have discovered nearly 300 new galaxy clusters and groups in the NDWFS Bootes field, nearly 100 of which are at 1 < z < 2. To date we have spectroscopically confirmed 7 of these high redshift clusters, spanning 1.06 < z < 1.41. In this regime clusters have the mean colors predicted by a passive evolution model with a high (z > 3) formation redshift. Deep follow-up IRAC and ACS data, currently being obtained for 18 of the z > 1 clusters, allows detailed studies of the mass assembly history in massive galaxies at this key epoch.