Conclusions.

1. Even though several possible aging mechanisms are well understood, the aging of resonators is still not well understood.
   
2. Aging performance, including variations with temperature and drive level, depends on the resonator design and fabrication technology.
   
3. Many processing deviations can degrade aging performance.
   
4. High temperature processing seems necessary (but not sufficient) for the production of low-aging resonators.
   
5. The SC-cut, and modern ultrahigh vacuum and high temperature fabrication techniques have resulted in resonators which achieve low-aging in a shorter period of time than the best resonators made a generation ago, however, the aging of the best modern resonators after extended periods is no better today than what was reported for the best resonators in the 1960s.
   
6. Accelerated aging studies are useful for process control. Using accelerated aging data for long term aging predictions is possible, but considerable work and expense are required to reduce the risk of error to acceptable confidence levels
   
7. The best reproducible long term aging rates seem to be a few times 10^-11 per day. Occasional resonators exhibit aging rates of a few times 10^-12 per day after extended periods.
8. Environmental changes can produce frequency changes that appear to be aging. This apparent aging is now called "drift."

Although the state-of-the-art in the long term aging of low-aging resonators has been on a plateau for more than a generation, there is no reason to believe that the factors responsible for limiting the achievable long term aging are insurmountable. The definitive experiments, in which all known aging mechanisms are minimized, are yet to be performed. It is the authors' hope that the review in this paper will assist future researchers in the design of experiments that result in significant improvements in aging.

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