Human glial fibrillary acidic protein (GFAP) is a 432-amino acid long polypeptide encoded by the corresponding gene on chromosome 17q21. It belongs to the type-III intermediate filaments and is responsible for maintaining the mechanical strength of astrocytes which support and regulate the blood-brain barrier. Moreover, GFAP is involved in fundamental and critical astrocytic functions like motility, proliferation, synaptic plasticity, myelination and responses to brain damage. GFAP is highly but not exclusively expressed in astrocytes in the central nervous system (CNS). GFAP has recently drawn attention due to its potential as a promising biomarker for several neurological disorders where it has been shown to have value in disease diagnosis as well as disease progression and treatment monitoring. Several studies have demonstrated elevated GFAP levels in Alzheimer’s disease (AD). Furthermore, blood GFAP levels seem to increase more than 10 years before symptom onset in genetic AD patients. In addition, blood GFAP levels might be able to predict the conversion from AD mild cognitive impairment to AD dementia. In multiple sclerosis (MS) GFAP levels vary by MS subtype and may be used as disease severity and progression biomarker.