Instrument rating

An Instrument Rating is required for a pilot to fly under Instrument Flight Rules (IFR) or in weather below the minimums for Visual Flight Rules (VFR).

In the U.S., the rating is issued by the FAA.

Instrument rating standards
To be eligible to pursue an Instrument Rating, the applicant must:
 * Hold at least a Private Pilot Certificate.
 * Pursuant to the requirement to hold the Private Pilot Certificate, the applicant must be at least 17 years old.
 * Be able to read, write, and converse fluently in English.
 * Hold a current FAA Medical Certificate, unless the Practical Examination is administered, in its entirety, in an FAA-certified Level D Flight Training Device.
 * Receive and log ground training from an authorized instructor (i.e. ground school course) or complete a home-study course using an instrument textbook and/or videos.

Ground training

 * Candidates for the instrument rating must be knowledgeable in IFR-related items in the AIM, the U.S. ATC system and procedures, IFR navigation, the use of IFR charts, aviation weather, requirements for operating under IFR conditions, recognition of critical weather, Aeronautical Decision Making (ADM) and Crew Resource Management (CRM).
 * Candidates must also pass the FAA instrument rating knowledge test with a score of 70% or better.

Flight experience and training

 * Accumulate flight experience per FAR 61.65:
 * Have 50 hours of cross-country flight time as pilot in command, which can include solo cross-country time as a student pilot. Each cross-country must have a landing at an airport that was at least a straight-line distance of more than 50 NM from the original departure point. Cross-country flight procedures must include at least one cross-country flight that is performed under IFR and consists of a distance of at least 250 NM along airways or ATC-directed routing, an instrument approach at each airport, three different kinds of approaches with the use of navigation systems (ILS, LOC, VOR, ADF, GPS)
 * The candidate also needs a total of 40 hours of actual or simulated instrument time, including 15 hours of instrument flight training from a Flight Instructor certified to teach the instrument rating (CFII)
 * Up to 20 hours of the instrument training may be accomplished in an approved flight simulator or flight training device if the training was provided by a CFII.
 * Within 60 days of the practical test, the candidate needs to log 3 hours of instrument training from a CFII in preparation for the test.
 * Receive and log training, as well as obtain a logbook endorsement from your CFII on the following areas of operation: preflight preparation, preflight procedures, air traffic control clearances and procedures, flight by reference to instruments, navigation systems, instrument approach procedures, emergency operations, and postflight procedures.
 * Successfully complete the instrument rating practical test (and oral and flight test), as specified in Practical Test Standards (PTS) for the instrument rating, which will be conducted by an FAA designated examiner.

Operations requiring an instrument rating
A pilot must have an instrument rating in order to act as Pilot in Command of a flight under IFR. The rating is also required:
 * When flying an airplane under Special VFR at night (helicopters are excepted from the regulation.)
 * When flying an airplane carrying passengers for hire, on flights in excess of 50 nautical miles or at night.

IFR currency requirement
Under FAR 61.57, to be eligible to fly in Instrument Meteorological Conditions (IMC) an IFR-rated pilot must accomplish and log at least the following IFR procedures under actual or simulated IMC every 6 months:
 * 6 instrument approaches
 * Holding procedures
 * Intercepting and tracking courses through the use of navigation systems

An Instrument Proficiency Check administered by a CFII within the last 6 months is another way of complying with the IFR currency requirement.

Reference

 * Federal Aviation Regulation 61.65

Reference: Wikipedia "Instrument Rating"