Early child development is an indeterminate period, but is generally considered to comprise the period between conception and 7 years of age (Irwin et al. 2007). This important life stage is described as a social determinant of health, because it exerts such a critical influence on the infant’s later health and development, to the extent that it affects inequalities across the population. This gave rise to the title of one report: ‘Early child development – a powerful equalizer’ (Irwin et al. 1997). Other reports have suggested the foundation years (Field 2010) are the most important period within that, referring to the first ‘1001 critical days’ from conception to aged 2, because of the extent of robust evidence showing the later effect of this very early period of life (Wave Trust 2012; All Party Parliamentary Group for Conception to Age 2 – The First 1001 Days 2015). This chapter focuses mainly on this very early period of life, but wherever the emphasis is placed, there is a clear consensus in the evidence that early childhood matters, not only to individual children and their families, but to the whole of society.