The Skills Imperative 2035 is a five-year strategic research programme which is investigating future skills needs, skills supply and skill development, with a particular focus on the 'Essential Employment Skills' (EES) that are projected to be most vital across the labour market in 2035. This paper investigates childhood skill development and gaps, with the focus in this stage of the programme examining the factors associated with children's development across a broader set of related cognitive and behavioural attributes, in line with the hypothesis that children's cognitive skills and socio-emotional behaviours are antecedents for their EES in early adulthood. These EES then, in turn, are likely to have a significant bearing on young people's ability to enter, or progress into, growing, predominantly professional, occupations. This paper is designed to be read in conjunction with its summary report, which summarises the research findings in this paper and relates them to the findings from previous papers in The Skills Imperative 2035 about future skills needs and gaps, particularly the growing demand for workers to utilise high levels of EES. This analysis draws on nationally representative birth cohorts and longitudinal studies, principally the Millennium Cohort Study (MCS) linked to data on the same individuals from the National Pupil Database (NPD). The authors also utilise data from the Understanding Society (USoc) study, a household-level study which five and eight year-olds over time, between 2011 and 2022, to explore how young people's behavioural outcomes and home environments have changed over time. The report explores the factors which contribute to cognitive and behavioural development up to age 17, examines how cognitive and behavioural skills develop throughout childhood and the relationship between young people's behavioural and cognitive outcomes, and then investigates how these skills develop across different development stages and how the importance of different factors changes as children get older. The report then briefly compares the behavioural difficulties of five and eight year olds between 2011 and 2022, to examine changes in skill levels over time, and then simulates the effects of improvements in young people's home and school environments on their cognitive and behavioural outcomes.