Mutations in RUNX1 and CBFB have long been recognized as important in hematological malignancies. Point mutations and deletions of RUNX1 are frequently found in myelodysplastic syndrome, myeloproliferative disease, and acute myeloid leukemia. Germline mutations in RUNX1 are associated with familial platelet disorder with predisposition to AML. In addition, as will be discussed in other chapters, both RUNX1 and CBFB are involved in recurrent chromosomal rearrangements in leukemia. More recently, roles for the non-mutated RUNX1 and CBFB genes have been identified in multiple leukemia subtypes. This chapter will discuss the roles of RUNX1 and CBFB, both in diseases caused by their mutations or deletions, as well as in the context of chromosomal rearrangements.