To amplify the target DNA it is necessary to cycle through these temperatures several times (25 to 40 depending on the application). Conveniently, this temperature cycling is accomplished by using a thermal cycler, a programmable instru- ment that can rapidly alter temperature and hold samples at the desired temperature for a set time. This automation is one of the important advances that led to PCR becoming widely accessible to many scientists and is covered in more detail in Chapter 3. Before thermal cyclers became avail- able, PCR was performed by using three water baths set to temperatures of typically 95°C, 55°C and 72°C, and reaction tubes in racks were moved manually between the baths.