DNA sequencing is the process of determining the order of the four chemical building blocks, called "bases," that make up the DNA molecule. The four bases are adenine, guanine, cytosine, and thymine. DNA sequencing may be used to determine the sequence of individual genes, larger genetic regions, full chromosomes, or entire genomes of any organism. DNA sequencing is also the most efficient way to indirectly sequence RNA or proteins via their open reading frames. DNA sequencing has become a key technology in many areas of biology and other sciences such as medicine, forensics, and anthropology.

There are several different methods available for DNA sequencing, each with its own characteristics, and the development of additional methods represents an active area of genomics research. One of the most common methods is Sanger sequencing, which involves copying the target DNA many times, making fragments of different lengths, and marking the ends of the fragments with fluorescent "chain terminator" nucleotides to allow the sequence to be determined. Another method is shotgun sequencing, which is designed for analysis of DNA sequences longer than 1000 base pairs, up to and including entire chromosomes. This method requires the target DNA to be broken into random fragments, and the sequences can be reassembled on the basis of their overlapping regions.

DNA sequencing has many applications, including identifying the particular type of cancer a patient has, which enables physicians to make better choices for treatment. DNA sequencing can also be used to determine which stretches of DNA contain genes and which stretches carry regulatory instructions, turning genes on or off. In addition, sequence data can highlight changes in a gene that may cause disease.