On 21 July 1969, during the first manned lunar mission, Apollo 11, the first retroreflector array was placed on the moon, enabling highly accurate measurements of the Earthmoon separation by means of laser ranging. Lunar laser ranging (LLR) turns the Earthmoon system into a laboratory for a broad range of investigations, including astronomy, lunar science, gravitational physics, geodesy, and geodynamics. Contributions from LLR include the three-orders-of-magnitude improvement in accuracy in the lunar ephemeris, a several-orders-of-magnitude improvement in the measurement of the variations in the moon's rotation, and the verification of the principle of equivalence for massive bodies with unprecedented accuracy. Lunar laser ranging analysis has provided measurements of the Earth's precession, the moon's tidal acceleration, and lunar rotational dissipation. These scientific results, current technological developments, and prospects for the future are discussed here.

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Analysis of 24 years of lunar laser ranging data is used to test the principle of equivalence, geodetic precession, the PPN parameters ␤ and ␥, and Ġ /G. Recent data can be fitted with a rms scatter of 3 cm. ͑a͒ Using the Nordtvedt effect to test the principle of equivalence, it is found that the Moon and Earth accelerate alike in the Sun's field. The relative accelerations match to within 5ϫ10 Ϫ13 . This limit, combined with an independent determination of ␥ from planetary time delay, gives ␤. Including the uncertainty due to compositional differences, the parameter ␤ differs from unity by no more than 0.0014; and, if the weak equivalence principle is satisfied, the difference is no more than 0.0006. ͑b͒ Geodetic precession matches its expected 19.2 marc sec/yr rate within 0.7%. This corresponds to a 1% test of ␥. ͑c͒ Apart from the Nordtvedt effect, ␤ and ␥ can be tested from their influence on the lunar orbit. It is argued theoretically that the linear combination 0.8␤ϩ1.4␥ can be tested at the 1% level of accuracy. For solutions using numerically derived partial derivatives, higher sensitivity is found. Both ␤ and ␥ match the values of general relativity to within 0.005, and the linear combination ␤ϩ␥ matches to within 0.003, but caution is advised due to the lack of theoretical understanding of these sensitivities. ͑d͒ No evidence for a changing gravitational constant is found, with ͉Ġ /G͉р8ϫ10 Ϫ12 /yr. There is significant sensitivity to Ġ /G through solar perturbations on the lunar orbit.

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remains static in an inertial frame. Without loss of generality we define an inertial frame (CM frame) with CM as its origin and let the CM, CE and CF coincide before mass redistribution. In CM frame, the coordinates of CE, CF and mass load are denoted as rCE, rCF and rload respectively. We define the coordinates of CF and mass load in an Earth-fixed reference frame (CE frame) with CE as its origin as rCF and rload. There are simple geometric and mass balance relations rload = rCE + rload, rCF = rCE + rCF (1)