Neurofilament light chain (NfL) is a biomarker for both disease progression and treatment response in multiple sclerosis (MS), but the search continues for additional biomarkers to distinguish between disease activity and progression. Serum glial fibrillary acid protein (GFAP) could be a useful complement to NfL in MS, although it is not ready for the clinic.NfL is a structural protein of neurons, while GFAP is a structure protein of astrocytes. NfL therefore reflects neuronal damage, while GFAP is an indicator of astrogliosis and astrocytic damage. GFAP has been shown to be increased in progressive MS and has been applied in traumatic brain injury and neuromyelitis optica spectrum disorder. Serum GFAP is a promising biomarker reflecting progression in MS and it is complementary to NfL.