Interest in the extent of mobility in individual earnings over time has increased greatly in recent years. New sources of data have become available, particularly with the establishment of panel, or longitudinal, surveys, and with the greater access to data contained in administrative records. The need for data on earnings mobility has become apparent in such areas as the design of pension schemes, where entitlements to benefits depend on a person’s earnings career and cannot be predicted solely from a cross-section picture of earnings. In the measurement of inequality, it has been recognised that a simple snapshot of the earnings distribution is inadequate and that lifetime equity depends on the extent of movement up and down the distribution. The distribution of earnings may look the same in two years, but there may have been a great deal of turnover. Are people persistently low paid, or are low earnings largely transitory?