Aspects of the Earth's variable speed of rotation, or variations in the length of day, and their geophysical consequences and causes are reviewed. Emphasis is placed on those areas which may benefit most from improved observations of the rotation rate. Seasonal changes in the length of day are primarily of meteorological origin. Zonal winds, in particular, play an important role, and year-to-year variations in the magnitude of the seasonal rotational characteristics provide information on the variability of the year-to-year atmospheric circulation. Changes observed since 1955 in the annual and semi-annual change in the length of day indicate a decreasing strength of the zonal circulation at these frequencies. Changes observed in the astronomical biennial term indicate that the biennial zonal winds propagate downwards to variable depths and that it is of variable period. Higher-frequency variations in length of day are also primarily of meteorological origin and will mask or interfere with other geophysical factors affecting the Earth's rotation, such as tides or earthquake caused changes in the inertia tensor. Thus improved observations of the variable rotation will have to be accompanied by improved global compilations of zonal winds so that the meteorological contribution can be evaluated with equal accuracy. Present compilations of wind data are inadequate for this. An area where satellite observations can make an important contribution to studies of the Earth's rotation concerns the separation of the secular tidal and non-tidal changes in length of day by studying the tidal perturbations in satellite orbits.