The present chapter provides an overview of the thermodynamic principles, historical evolution, and technological development of hydrogen liquefaction processes. The important issues to be overcome, such as high energy consumption, ortho–para hydrogen conversion, inversion temperature limitations, and boil-off losses, are discussed step by step. Classic liquefaction cycles, including the Linde–Hampson and Claude cycles, as well as their advanced variants, are reviewed in terms of efficiency and specific energy consumption. The role of pre-cooling, recuperative heat exchange, expanders, and auxiliary refrigeration systems is critically examined.