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Minimum requirements prescribed by the FAA that must be met:
Hold at least a current private pilot certificate or be concurrently applying for a private pilot certificate with an airplane, helicopter, or powered-lift rating appropriate to the instrument rating sought.
Be able to read, speak, write, and understand the English language.
At least 50 hours of cross-country flight time as pilot in command. At least 10 of these hours must be in airplanes for an instrument-airplane rating.
A total of 40 hours of actual or simulated instrument time on the areas of operation listed in 61.65(c).
At least 15 hours of instrument flight training from an authorized instructor in the aircraft category for the instrument rating sought.
Aircraft Owners and Pilots Association AOPA can also be extremely helpful to get more information about the requirements.