Multiple sclerosis (MS) is a chronic inflammatory and neurodegenerative autoimmune disease of the central nervous system (CNS). In the majority of patients, MS manifests as a relapsing-remitting (RR) disease, which may evolve over time into secondary progressive (SP) disease, leading to irreversible disability. The exact pathogenic mechanisms underlying disease progression are unknown. However, compartmentalized chronic inflammation, including activation of microglial cells, likely plays a central role. Neuropathological studies have demonstrated that activated microglia are present at the rims of chronic active lesions as well as diffusively in the normal-appearing white matter (NAWM). Microglial activation has been shown to correlate with disease severity and predicts disease progression. Positron emission tomography (PET) imaging using radioligands binding to translocator protein (TSPO) enables in vivo quantification of microglial activation in the brain, and TSPO-PET is considered a marker of inflammatory activity in MS. This thesis consists of three studies. Study I assessed, which demographic, clinical, and paraclinical MS disease-related parameters are associated with later TSPO-PET measurable microglial activation. Study II evaluated the effect of sex on [ 11C]-PK11195 binding in people with MS (pwMS) and in healthy individuals. Study III explored the usability of various imaging and serum biomarkers in predicting conversion to SPMS. In these studies, a higher number of WM lesions on diagnostic brain MRI, a higher immunoglobulin (IgG) index, and an early increase in the Expanded Disability Status Scale (EDSS) score were associated with increased microglial activation an average of 12 years later. Compared to women, [ 11C]-PK11195 binding was more prominent in both healthy men and male pwMS. Increased microglial activation, as quantified by TSPO-PET, elevated serum glial acidic fibrillary protein (GFAP) concentration, and thalamic atrophy predicted SPMS conversion. This thesis demonstrates an association between early clinical factors and later microglial activation that is detrimental to disease progression, and suggests that TSPO-PET, along with serum GFAP concentration and thalamic volume measurement, may be useful in assessing disease prognosis in pwMS.