IQ tests and achievement tests do not capture non-cognitive skills — personality traits, goals, character and motivations that are valued in the labour market, in school and elsewhere. For many outcomes, their predictive power rivals or exceeds that of cognitive skills. Skills are stable across situations with different incentives. Skills are not immutable over the life cycle. While they have a genetic basis they are also shaped by environments, including families, schools and peers. Skill development is a dynamic process. The early years are important in shaping all skills and in laying the foundations for successful investment and intervention in the later years. During the early years, both cognitive and non-cognitive skills are highly malleable. During the adolescent years, non-cognitive skills are more malleable than cognitive skills. The differential plasticity of different skills by age has important implications for the design of effective policies.