Abstract: A thermocycler is provided, that in one embodiment has separate heat exchangers for each thermocycler target temperature, and a cold boost heat exchanger and a hot boost heat exchanger. Fluid conduit is used to circulate fluid through the appropriate heat exchanger and through a sample holder containing at least one sample. Each heat exchanger has sufficiently high thermal mass to be susceptible to being adjusted to and maintained at a constant temperature, but the remaining components of the heat exchanger are preferably of low thermal mass so as to improve efficiency of the system. A small volume of circulating fluid is preferably used. In use, the temperature of the samples is increased or decreased rapidly by first passing the circulating fluid to the hot boost or cold boost heat exchanger, followed by passing the circulating fluid to the appropriate target temperature heat exchanger. A controller is used to control the heating, cooling, and duration of the various aspects of a thermocycler cycle.