Working memory (WM) und fluid intelligence (Gf) are both key abilities in many cognitive development areas. Previous studies showed that both constructs are substantially related. However, WM consists of different aspects and it is not clear which of these aspects underlies this relation. Moreover, previous studies investigating the development of the neural basis of verbal WM have neglected the fact that WM and Gf are related. Accordingly, the main goal of the two studies presented in this dissertation was to gain a deeper understanding of the relation between WM and Gf. In addition, we aimed to investigate how age and Gf combine to affect WM performance and WM-related brain activity in middle and late childhood. The main question addressed in this umbrella paper was driven by the question of how Gf and WM develop, which was incorporated in both studies. The results revealed that WM and Gf improve with increasing age. Moreover, results showed that different WM aspects promote the development of WM as well as of Gf. However, also individual differences in Gf seem to foster the development in WM. Hence, the relation between WM and Gf seems to go in both directions. In addition, neural results revealed an age-by-Gf interaction effect on WM-related brain activation, indicating that also functional brain differences contribute to the development of WM and Gf. The results are discussed in terms of neural efficiency and in terms of theoretical and practical implications.