In the field of neuromodulation, we have observed an increase in the popularity of approaches that combine transcranial electrical stimulation (tES) with additional methods to establish, in vivo, the neurophysiological consequences of a given experimental or therapeutic manipulation. Research in the development of multimodal approaches has provided new methods that can be combined with tES to study brain functions. This chapter aims to introduce the reader to some basic principles of this multimodal approach. We begin with a brief definition of multimodal association and a description of its advantages. Afterwards, we provide a more specific description of how we can combine tES with electroencephalography (EEG). We show that EEG is a feasible and reliable way to track electrophysiological changes induced by tES, deepening our understanding of the mechanisms of action of this tool and revealing the key role of several stimulation features. In neuropsychiatric diseases, a combined tES-EEG approach may allow the prediction of clinical responses to tES, the discrimination of responders from non-responders, allowing improvements in the efficacy of tES and the tracking of tES-induced neuroplastic changes associated with recovery.