The payload of the Spitzer Space Telescope is the Cryogenic Telescope Assembly (CTA), a thermal and optical system that houses the science instruments and provides them a 1.2 K thermal sink. It also provides the 0.85-meter telescope, which is controlled between 5 K and 12 K to achieve the appropriate low photon background for the instruments while conserving helium. This cryogenic system supplies cooling through a combination of passive radiation and controlled vapor flow from a superfluid helium cryostat. Unlike previous cryogenic space infrared telescopes, the CTA allows the users to selectively cool Spitzer for particular science operations. Each science opportunity is both a benefit to the astronomical community and a cost to Spitzer's cryogen lifetime. CTA allows these benefits and costs to be weighed. Launched warm in August 2003 with 49 kg of helium, the CTA has been performing superbly with a current helium loss rate of only 9 kg per year after the initial cool-down period. Remaining helium is measured periodically so that mission lifetime can be accurately determined. Due in large part to the success of Spitzer, various aspects of the warm launch design have become the standard for future cryogenic space telescopes. This report describes the CTA and provides flight performance data for the cryogenic system.