The development of specialized cryogenic systems which can access temperatures below 1 K is an exciting challenge for physicists and engineers. And as the number of scientists interested in the thermodynamics of producing ultra-low temperatures is steadily decreasing, many more are becoming interested in using refrigerators capable of reaching such temperatures as a tool to perform a wide range of experiments. Sometimes certain phenomena only occur at low temperature, such as the superfluid phases of 3He and 4He. In other cases, the physical property being studied becomes less complex and more amenable to theoretical interpretation, for example, in non-metallic crystals. The range of applications of these systems is surprisingly diverse and with most refrigerators being used in fundamental research it is inevitable that new requirements develop continuously. The designers of such products need to liaise with potential users and to combine physics and engineering to develop cost-effective and thermodynamically efficient systems. This brings the ultra-low-temperature regime within reach of specialists from other fields.