Pyrosequencing is a DNA sequencing technique that is based on the detection of released pyrophosphate (PPi) during DNA synthesis. In a cascade of enzymatic reactions, visible light is generated that is proportional to the number of incorporated nucleotides (Fig.1). The cascade starts with a nucleic acid polymerization reaction in which inorganic PPi is released as a result of nucleotide incorporation by polymerase. The released PPi is subsequently converted to ATP by ATP sulfurylase, which provides the energy to luciferase to oxidize luciferin and generate light. Because the added nucleotide is known, the sequence of the template can be determined. The nucleic acid molecule can be either RNA or DNA. However, because DNA polymerases show higher catalytic activity than RNA polymerases for limited nucleotide extension, efforts have been focused on the use of a primed DNA template for pyrosequencing. Standard pyrosequencing uses the Klenow fragment of Escherichia coli DNA Pol I, which is a relatively slow polymerase (Benkovic and Cameron 1995). The ATP sulfurylase used in pyrosequencing is a recombinant version from the yeast Saccharomyces cerevisiae(Karamohamed et al. 1999a) and the luciferase is from the American firefly Photinus pyralis. The overall reaction from polymerization to light detection takes place within 3–4 sec at room temperature. One pmol of DNA in a pyrosequencing reaction yields 6 × 1011 ATP molecules which, in turn, generate more than 6 × 109 photons at a wavelength of 560 nanometers. This amount of light is easily detected by a photodiode, photomultiplier tube, or a charge-coupled device camera (CCD) camera. There are two different pyrosequencing strategies that are currently available: solid-phase pyrosequencing (Ronaghi et al. 1996) and liquid-phase pyrosequencing (Ronaghi et al. 1998b). Solid-phase pyrosequencing (Fig. 2) utilizes immobilized DNA in the three-enzyme system described previously. In this system a washing step is performed to remove the excess substrate after each nucleotide addition. In liquid-phase pyrosequencing (Fig.3) apyrase, a nucleotide-degrading enzyme from potato, is introduced to make a four-enzyme system. Addition of this enzyme has eliminated the need for solid support and intermediate washing thereby enabling the pyrosequencing reaction to be performed in a single tube. These formats are described in detail in this review.