Most solid tumors are aneuploid, and many tumor cells persistently mis-segregate whole chromosomes at an elevated rate in a process called chromosomal instability (CIN). CIN occurs due to loss of chromosome segregation fidelity during mitosis, however, the cause of these chromosome segregation errors in tumors remains unknown. Here I show that the most common mitotic defect in multiple cancer cell lines with CIN are lagging chromosomes in anaphase caused by a specific type of malattachment of microtubules to kinetochores called merotely. Overexpression of microtubule depolymerizers Kif2b and MCAK in cell lines with CIN reduces levels of merotely and chromosome lagging and restores faithful chromosome segregation. In addition, diploid cells can be induced to mis-segregate chromosomes at levels similar to cells with CIN by persistently elevating the levels of merotely, however, aneuploid cells produced from these diploid cells do not propagate efficiently and the population remains diploid due to growth arrest in the aneuploid cells caused by stabilization of p53 and p21. When p53 or the stress kinase p38 is inhibited in diploid cells that have been induced to mis-segregate chromosomes, aneuploid cells can propagate and are maintained in the population. In the diploid HCT116 colon cancer cell line, deletion of p53 combined with several generations of induced persistent chromosome missegregation is sufficient to produce aneuploid, CIN clones. Subclones derived from these clones remain chromosomally unstable despite no longer being induced to missegregate chromosomes, which suggests aneuploidy itself causes self-perpetuating CIN. Finally, when merotely is elevated through drug washouts or depletion of MCAK, Kif2b, CENP-E, or Kif18a, there is a high incidence of both lagging chromosomes and mis-segregation, but most chromosomes mis-segregate without evidence of lagging. Most lagging chromosomes segregate to the correct daughter cell, where they form micronuclei which can be reincorporated into the main nucleus during the next round of mitosis. Thus lagging chromosomes are an indicator of high levels of merotely and missegregation, but not necessarily the cause. Together, the data in this thesis suggest elevated levels of mis-segregation must combine with mutations that allow for tolerance of nondiploidy to generate the levels of aneuploidy seen in tumors.