Cancer is a heterogenous disease which can acquire multiple hallmarks throughout its multistep development in human tumors. The six main hallmarks are evading growth suppressors, resisting cell death, sustaining proliferative signaling, enabling replicative mortality, activating invasion and metastasis, and inducing angiogenesis (sprouting of new blood vessels). When looking at both cellular organization and interactions between (cancer) cells and the tumor microenvironment, all these hallmarks can be observed by using high-resolution imaging. 3D electron microscopy is a powerful diagnostic technique in cancer research and one of the most accurate ways to get an overview of the entire ultracellular structure on the nanoscale.