Countries that exhibit similar levels of overall income mobility may contrast greatly in terms of the underlying forces driving mobility processes. A low level of income mobility may be a result of market inefficiency (lack of opportunity) in one country, while a result of generous welfare (stability) in another (Jantti and Jenkins, 2015). It concentrates on income changes over (working-age)3 individuals’ “life course”, ranging from medium-term income mobility (four years) to longer-term income changes (nine years). The chapter first identifies how taking account of income mobility changes our assessment of existing levels of income inequality.