This chapter focuses on the concept of variability in response to non-invasive brain stimulation (NIBS) techniques, with a particular emphasis on transcranial direct current stimulation (tDCS). It explores both inter-individual and intra-individual variability, analyzing the presence of responders and non-responders to stimulation, as well as the different statistical methods employed to assess this variability. While intra-individual variability appears to be lower than inter-individual variability, it still presents challenges in interpreting NIBS results.Furthermore, the chapter discusses the factors that influence variability in NIBS studies. These factors can be categorized into non-modifiable physiological factors (such as sex, age, genetics and anatomical features), modifiable factors (including medical and non-medical substances), technical factors (e.g., stimulation parameters), and statistical aspects.By identifying the contributing factors and tailoring stimulation protocols to individual needs, researchers can optimize the effectiveness of NIBS interventions. However, due to the complexity and multitude of factors involved, more research is needed to fully elucidate the mechanisms underlying this variability in NIBS.<br/
