This PhD thesis focuses on using brain stimulation to increase people’s arithmetic skills and to examine the underlying neural processes. Transcranial electrical stimulation (tES) is a technique capable of mapping the relationship between behaviour and brain function. A major limitation of brain stimulation is that it does not take into account differences between people. In the first two chapters, it becomes clear that individual differences are not accounted for in studies using brain stimulation as a numerical intervention. The next chapter examines several studies that aim to improve learning in dyscalculia and dyslexia. Two other studies show that there are both important behavioural markers (baseline numeracy) and neural markers (excitation/inhibition levels) for the efficacy of tES. One of these studies also shows the importance of personalising brain stimulation, thereby emphasising individual differences using machine learning. The final chapter shows that the way researchers analyse electrophysiological (EEG) data matters and is important when drawing conclusions in relation to behaviour. In short, brain stimulation is an effective method to influence numeracy and learning processes. Individual differences are also important.