The trillions of cells in the human body were created by countless cycles of cell growth and division during development. These cycles continue during our lifetime to renew and repair tissues. Uncontrolled cell division cycles, however, cause cancer. A critical event for healthy cell division is the proper duplication and segregation of all chromosomes. We use a range of molecular biology, tissue culture and imaging technologies to understand 1) how error-free chromosome segregation is ensured molecularly; 2) how errors in chromosome segregation impact tumor development; 3) what the evolutionary origins and diversity of these processes are in eukaryotes.