In early 1988 the Cosmic Background Explorer will be placed into a 900 km orbit from which the spectrum of diffuse radiation will be measured from 1 micron to 1 cm. A critical component of the observatory is the 650 liter superfluid helium dewar, which will house the two cryogenic instruments: a far infrared absolute spectrophotometer and a diffuse infra-red background experiment. Fabrication and testing of the dewar is complete, and measured performance exceeds requirements. The four-month test program included four major phases: (1) filling the dewar with superfluid helium and verifying basic functions, primarily fluid management and instrumentation, (2) vibration testing to verify the structural math model and prove Shuttle launch compatibility, (3) thermal performance testing to verify the thermal math model and prove the orbital lifetime requirement will be met, and (4) testing to prove that in a simulated orbital environment the aperture cover will eject properly. No significant flaws in the dewar performance were encountered during testing. In laboratory conditions the "hands-off" helium loss rate was measured at 0.75 percent per day, and the bath temperature (with pumping) was 1.65K. The boil-off rate during ground testing is about four times greater than during orbital operation because of differences in configuration and boundary conditions. To verify orbital performance through ground testing, it is therefore necessary to use a math model which is rigorously validated by test data. Testing was designed specifically to accomplish this. Orbital cryogen lifetime is predicted at 14 months, compared to 10 months required with a design goal of 12 months. Major design features, test results, thermal math model correlation, and orbital performance predictions are discussed.