The general principles underlying the practical design of Joule‐Thomson hydrogen liquefiers, utilizing the compressed hydrogen commercially available in cylinders, are discussed. The theory of heat interchangers is applied to these liquefiers and design formulae and data are presented. A typical design for the regenerative interchanger of a small scale liquefier is given. The liquid nitrogen requirements, expansion valve design, charcoal trap construction, and other practical aspects of these liquefiers are also discussed.