Uracil is a ribonucleotide found in both DNA and RNA, with the main difference between the two being the presence of thymine in DNA and uracil in RNA. Although thymine and uracil are similar in function and can form the same base pairs with adenine, the presence of uracil in DNA can affect DNA stability and modulate cell-specific functions. Without repair mechanisms to remove uracil from DNA, cytosine deamination can occur, resulting in gene drift that is not tolerable in organisms. While the deamination of cytosine in DNA signals damage, a corresponding deamination in RNA would yield normal RNA constituents. To correct this, uracil DNA glycosylases detect and remove uracil bases from uracil-containing DNA, but not natural thymine-containing DNA. The mechanisms of uracil incorporation into DNA, its roles in DNA, cellular mechanisms to detect and remove uracil, and the clinical utility of uracil in DNA will be discussed in this chapter.