Arginase is a ureohydrolase that synthesizes L-ornithine and urea from L-arginine. In mammals and amphibians, who excrete excess nitrogen through the urea cycle, it exists in two isoforms. Arginase 1 is a cytosolic enzyme mostly expressed in liver where it contributes in the urea cycle. Arginase 2 is a mitochondrial enzyme present in different tissues. L-ornithine is a non-proteinogenic amino acid and a precursor for the synthesis of polyamines, proline, glutamate and glutamine. Polyamines are molecules that promote cell proliferation and proline is a precursor for collagen synthesis. Regulation of arginase activity enables regulation of cell proliferation and tissue repair. Nitric oxide synthases use L-arginine for the synthesis of nitric oxide, an important signalling molecule in controlling vasodilatation and blood flow. Various cardiovascular, renal and neurovascular diseases are supported by lowered nitric oxide syntesis and increased arginase expression and activity. Increased arginase activity is present in tumors and chronic inflammations and suppresses immune response as well. Therapies using arginase inhibitors provide great potential in treating many diseases encouraged by disrupted arginase expression and activity.