**1. Introduction**

Neuro-oncology is a great challenge for neurosurgeons from two perspectives: first, in some types of tumours, gross total resection (GTR) is the best predictor of outcome in terms of life

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expectancy [1,2]; second, a main goal of every surgery is to avoid introducing new iatrogen‐ ic lesions. The relative weight of every one of these principles can be changed based on individual considerations of the type of tumour, the structures affected, the life expectancy and even the social considerations of each patient. These features are particularly relevant to patients suffering from high-grade gliomas, for whom survival is directly related to the degree of tumour removal. Therefore, to maintain an adequate quality of life, the primary goal of surgery is to achieve GTR without compromising neurological function.

Central nervous system tumours are relatively common in adults; they are the second most common form of cancer and the most common type of solid tumour in children. Although more than half of these tumours are benign, they can cause substantial morbidity. The most common tumours in adolescents and adults aged 15–34 years are gliomas and meningiomas [3]. Glioblastoma multiforme (GBM) is the most common type of glioma. Meningiomas derive from meningothelial cells and comprise approximately 20 percent of primary brain tumours. GBM are more commonly located in the supratentorial region, with the frontal lobe being the most common site [3].

Advances in surgical techniques, such as intraoperative neurophysiological monitoring (IONM), intraoperative magnetic resonance imaging (MRI), diffusion tensor imaging (DTI), stereotactic guidance and fluorescent-guided resection (FGR), have facilitated the delinea‐ tion of tumour borders and can aid in optimizing safe surgical resection [4–6].

Neurosurgery can be considered a radical method to treat some illnesses and can seriously damage the nervous system (NS). These injuries may not be apparent by visual inspection by the surgeon in the operating room but subsequently evolve into a definite lesion [7]. To avoid deleterious effects, such injuries can be detected during their initial development by IONM. Therefore, IONM is a powerful set of techniques that provide increased functional knowl‐ edge during a surgical operation, resulting in the safer removal of a radical tumour [8,9].

The operating room is an aggressive environment to perform recordings due to the presence of several sources of noise. Therefore, it is very important to identify the source of electro‐ magnetic noise and to determine how to manage it. Unfortunately, this subject is beyond the scope of this chapter, but we refer the reader to Pastor J, 2014, [10] for a detailed discussion of these topics.

In this chapter, we review the most relevant and frequently performed IONM techniques. We are especially interested in the clear and concise exposition of the methodological peculiari‐ ties, the fields of application and the flaws associated with the different techniques dis‐ cussed, with a focus on practical applications. Therefore, we show examples of real operations performed at our institution.
