The rotation of the solid Earth, as monitored from observatories fixed on the Earth's crust, is not constant. The measurements reveal minute but complicated changes of up to several parts in 108 in the speed of the Earth's rotation, corresponding to several milliseconds in the length of the day (LOD) and even larger variations in polar motion. Earth studies have embarked on a new era with the advent of highly accurate space geodetic techniques and the availability of complementary geophysical data sets. Techniques utilized include laser-ranging to the Moon and artificial satellites, and very long baseline interferometry (VLBI). Comparisons between techniques indicate that Earth rotation is routinely determined at the 0.05–0.10 millisecond level (approximately 2–5 cm at the equator), with higher accuracy in some cases. Geophysically interesting variations are detectable at these levels. The analysis and understanding of these phenomena draw upon and contribute to meteorology, oceanography, astronomy, celestial mechanics, seismology, tectonics, and geodynamics.