The enzymes responsible for epigenetic modifications are key regulators of gene expression, ultimately influencing cell fate and function during normal and oncogenic biological processes. These modifications can occur on histone proteins of the nucleosome to dynamically regulate chromatin structure at active and silenced gene loci. Methylation of histone H3 lysine 79 by Dot1 (disruptor of telomeric silencing) was first observed in yeast and serves as a marker for active transcription. Yeast Dot1 is a regulator of telomeric silencing, DNA damage repair, and the meiotic pachytene checkpoint. Additionally, Dot1 activity is regulated by the PAF complex, Rad6-Bre1 ubiquitination of H2B, and charge-based interaction with histone H4. Despite extensive research on Dot1, the role and regulation of mammalian homolog DOT1L (Like) is unclear. In this dissertation, I aim to understand the biological functions of DOT1L in both normal development and cancer. In mouse, DOT1L has been linked to embryogenesis and erythropoeisis; however, the molecular mechanisms underlying these two processes require further elucidation. Germ-line disruption of DOT1L resulted in embryonic lethality with cardiovascular defects. In these studies, I demonstrate that DOT1L is required for normal heart function. Cardiac-specific knockout of DOT1L causes dilated cardiomyopathy (DCM), which can be rescued by ectopic expression of minidystrophin. Mechanistically, DOT1L directly regulates transcription of dystrophin, which is critical for maintaining sarcolemma integrity and proper heart function. Finally, I provide evidence suggesting that malfunction of DOT1L activity may be a contributing factor in human DCM. Previously, DOT1L has been linked to leukemogenesis caused by chromosomal rearrangements of the MLL gene, encoding a histone H3 lysine 4 methyltransferase. Specifically, DOT1L enzymatic activity is required for MLL-AF10, MLL-ENL, and MLL-AF4 in vitro leukemic transformation. In these studies, I sought to expand the repertoire of MLL-fusion oncoproteins that utilize DOT1L by investigating MLL-AF9 leukemogenesis. Through in vitro and in vivo assays, I demonstrate that initial transformation by MLL-AF9 and maintenance of leukemic stem cell identity require DOT1L activity. These data support a universal mechanism involving mis-targeting of DOT1L and H3K79 hypermethylation for the up-regulation of leukemia-associated genes. Thus, DOT1L serves as a promising candidate for targeted therapeutics to treat MLL-related leukemias.