  The inexorable miniaturisation of technologies, the relentless drive to
improve efficiency and the enticing prospect of boosting performance through
quantum effects are all compelling reasons to investigate microscopic machines.
Thermal absorption machines are a particularly interesting class of device that
operate autonomously and use only heat flows to perform a useful task. In the
quantum regime, this provides a natural setting in which to quantify the
thermodynamic cost of various operations such as cooling, timekeeping or
entanglement generation. This article presents a pedagogical introduction to
the physics of quantum absorption machines, covering refrigerators, engines and
clocks in detail.
