It has been proposed that executive functions develop in a hierarchical fashion, such that early, simple abilities seen already during the first year of life become increasingly coordinated with development, thereby enabling the emergence of more complex abilities. Although this hierarchical model has received support from empirical studies comparing executive function task performance across age groups, necessary support from longitudinal studies taking an individual differences perspective on development is missing. In addition, the model stresses the importance of attention in executive function development, but we do not know in what way attention contributes to the continued development once the earliest forms of simple functions have emerged. Using a longitudinal design, the present study investigated the relations between individual differences in simpler forms of executive functions as well as sustained attention at age 12 months and more complex executive functions at 24 and 36 months. The results indicated partial support for the hierarchical model, with infant inhibition being predictive of working memory in toddlerhood. In addition, at 12 months, sustained attention contributed to the development of toddler executive functions via the simple executive functions. This suggests that by this age, sustained attention has become an integrated part of early, simple executive functions. Copyright © 2015 John Wiley & Sons, Ltd.