The faithful propagation of genetic information is one of the most fundamental problems in cell biology. Crucial to this process is the centromere, a special chromosomal locus defined epigenetically by the presence of CENP-A nucleosomes. CENP-A recruits the kinetochore protein complex which forms stable attachments to spindle microtubules for proper chromosome segregation in cell division. The kinetochore has been studied by electron microscopy since the middle of the last century, but a detailed understanding of its architecture has been hindered by artifacts in traditional electron microscopy sample preparations. In this thesis, we use cryo-electron tomography (cryo-ET) to overcome these obstacles. Cryogenic sample preparation involves rapid freezing of samples, capturing the structures of interest in their native conformations suspended in vitreous ice. Cryo-ET then makes it possible to visualize the 3D architecture of complex biological structures such as human chromatin. Here we use cryo-ET to directly visualize the 3D architecture of the human kinetochore on intact mitotic chromosomes. We find a distinctive chromatin architecture: kinetochores that consist of clustered protein complexes within chromatin clearings which delineate them from surrounding chromatin. Individual 20-25 nm kinetochore complexes are most consistent with a single copy of the constitutive centromere associated network (CCAN) associated with a CENP-A nucleosome. We find that the CCAN components CENP-C and CENP-N are each required for the integrity of inner kinetochore complexes, and CENP-C is also required to maintain the chromatin clearing, indicating a role in higher-order centromere organization. We also visualize the fibrous corona, a structure that forms at kinetochores lacking microtubule attachments both to promote capture by the spindle and to amplify spindle assembly checkpoint signaling until attachments are complete. The scaffold of the expanded corona consists of >1 micron parallel arrays of ~15 nm fibrils. Our findings provide new insights into the spatial differentiation of centromeric chromatin and provide an architectural framework to understand the organization of kinetochore complexes in context on mitotic chromosomes.