In this chapter, we consider children's early language acquisition as it develops alongside three other core cognitive competencies: children's understanding of objects, children's understanding of the thoughts and feelings of others (i.e. their theory of mind), and children's knowledge of numbers and mathematical principles. We argue that while early language acquisition is initially supported by these three other competencies, the acquisition of words transforms them by providing a method for children to communicate and organise the information obtained through them. We do this by describing the key milestones occurring within each of the competencies and the genetic and environmental factors which place them at risk, with findings derived from a systematic review of cohort studies in English-speaking countries. A key message is that genetic and environmental circumstances are inextricably linked in predicting early cognitive development, and that some factors are reliably stronger in predicting poor outcomes than others. Additionally, there is clear evidence that income-related disparities are reliably measurable from the age of 3 and then steadily increase throughout the remainder of childhood.