This book is meant for laboratory workers who for one reason or another have a need to cool something down to temperatures below that of liquid nitrogen - notably to 4. 2°K and below. It does not deal with experimental techniques at low temperatures, but I have tried to bring the reader face to face with the brutishrealities of the necessary hardware. As weIl as giving information about sources of supply of equipment, I have gone into so me detail about how some of it can be made in laboratory workshops for the sake of those who are short of money but blessed with competent technical support. So far as highly specialized items such as liquefiers, refrigerators, refrigerant containers, cryostat dewars, etc. , are concerned, I have included aIl sources of supply which I have got to he ar of; in the case of more generaIly available equipment only representative sources of known reliability have been quoted. Any omissions or errors must be put down either to my own ignorance, stupidity, or lack of will toget about the world, or perhaps to the difficulty I have had in extracting information from manufacturers. However, most have gone to great trouble to help, and I hope I have done them justice. Brought up to work indifferently in inches and centimetres and perched between the opposing puIls of the USA and Europe, I have used a mixture of units which may shock the purist.