Vacuum‐tight cryogenic containers can easily be made from Mylar plastic sheet and metal, or from Mylar alone, for use in vacuum‐insulated liquid hydrogen target assemblies. Mylar is bonded very firmly to metals or to itself by an Epon‐Versamid mixture or by Armstrong A‐4. These adhesives maintain strong vacuum‐tight joints throughout the temperature range from 300 to 4°K. Mylar sheet has also proved satisfactory as vacuum‐window material for beam ports. Fabrication of these structures is described in detail, and explosion safety measures are mentioned. Some examples are given of structures which have been used or tested, with data on the ultimate strength of each expressed as the internal pressure required to rupture it. Helium leak testing must be carried out at 77°K or below, because Mylar is relatively impermeable to helium only at low temperatures. The primary electron beams commonly obtained from electron linear accelerators are sufficiently intense to cause Mylar beam windows to fail under vacuum loading, probably because of radiation damage. However, the authors know of no case of radiation damage to Mylar sustained in exposure to secondary beams from high‐energy accelerators.