The key element in producing an efficient low temperture cryocooler is the performance of the regenerator. It must have good heat transfer characteristics while providing low pressure drop, especially for compact high speed coolers. The axial thermal conductivity must be low to reduce losses but provide good radial conductivity for flow uniformity and maximum use of all material. At low termperatures, the heat capacity of the regenerator material must be high to achieve these low temperatures. The geometry of the regenerator material will be investigated to meet the requirements and minimize losses while providing the necrssary characteristics for efficient cryocooler performance. Altermative fabricaiton methods and materials, including rare earths will also be investigated to take advantage of the improved low temperature material properties.