Abstract: Protein arginine methylation is a posttranslational modification that is carried out by enzymes known as protein arginine methyltransferases (PRMTs). PRMTs serve a variety of functions inside the cell, ranging from the regulation of transcription to cell signaling. I have identified novel eukaryotic PRMTs, including human PRMT8 and trypanosome TbPRMT5. In addition, I characterized several protein arginine methyltransferases to determine the type of enzymatic activity that they catalyze, as well as their substrate specificity. I characterized PRMT8 as a unique enzyme because it is both tissue specific (brain) and localized to the plasma membrane of cells via an N-terminal myristoyl modification. I demonstrated that the unique N-terminal domain may act to regulate PRMT8 activity. I have found arginine methylation in RNA-binding protein substrates, including Fmrp (the gene product of FMR1, the target of the Fragile X mental retardation syndrome), spliceosomal cytoplasmic Sm D2 and Sm E proteins, and the mitochondrial RNA-binding protein Rbp16 protein, a substrate of the newly identified trypanosome TbPRMT5. I have demonstrated methyltransferase activity in Hsl7 (H istone synthetic lethal 7), a protein involved in cell cycle morphology in the budding yeast Saccharomyces cerevisiae. Two chapters of this thesis are devoted to the detection of in vitro methyltransferase activity for Hsl7 with histone H2A, where both monomethylarginine and symmetric dimethylarginine were found under different conditions. Finally, I have shown that arginine methylation is a posttranslational modification that can be reversible when the enzyme peptidylarginine deiminase 4 (PAD4) converts monomethylarginine to citrulline in histones H3 and H4.