This chapter describes the extent to which single mothers report lower levels of well-being relative to their married counterparts. It recognizes the multifaceted nature of well-being and examines three different measures of well-being: happiness, physical health, and emotional health. The chapter addresses the limitations by using data from two rounds of new survey that includes a large oversample of single mothers. The data includes information on multiple dimensions of emotional and physical well-being including happiness, depressive symptoms, and self-rated health. The chapter contains information on many of the factors thought to contribute to lower levels of well-being among single mothers, including mothers' own socio-demographic characteristics, economic deprivation, and work-family stress. New data collected by the Japan Institute for Labour Policy and Training provides a valuable opportunity to which single mothers are indeed disadvantaged relative to their married counterparts, whether degree of disadvantage depends on the dimension of well-being considered, and factors that shape the well-being of single mothers.