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Pyrimidine nucleotides represent one of the most fundamental of cellular components. They are the building blocks for the direct synthesis of DNA and RNA that function in information storage and retrieval within the cell, but they also participate in the metabolism of a large number of other cellular components from sugar interconversion to cellular polysaccharides to glycoproteins and phospholipids. Thus, the metabolism of pyrimidine nucleotides and their intracellular pool sizes influence vast areas of normal cellular metabolism. The first pyrimidine, UMP, is synthesized by a de novo pathway that appears to be mechanistically invariant in all organisms. UMP is then further modified to form other pyrimidines. Breakdown of deoxyribo- and ribonucleic acids, the main sink for pyrimidine nucleotides, allows pyrimidines to be reutilized for resynthesis of these important cellular components. Pyrimidines are salvaged by converting the modified components into the free base, uracil for reutilization. Finally, pyrimidines are degraded into simple cellular metabolites permitting reutilization of nitrogen and carbon from pyrimidine ring systems into cellular metabolic pools. The regulation of pyrimidine metabolism is tightly controlled in plants. Additionally, plants produce toxic secondary metabolites derived from pyrimidines for use as defense compounds.