In this chapter, we introduce the principles of intraoperative monitoring and mapping techniques. We explain evoked potential monitoring during supratentorial surgery under general anesthesia and provide a detailed description of cortical and subcortical mapping techniques, including continuous dynamic mapping via a surgical instrument. Awake craniotomies and the Penfield technique are discussed as well. The main goal is to give the reader an understanding of all these techniques in the neurosurgical context. In particular, we evaluate how different warning criteria can provide functional guidance to achieve a “maximal safe” tumor removal during the surgery. Finally, some current research topics such as the evaluation of brain connectivity by paired pulse paradigms and cortico-cortical evoked potentials are presented.