Major depressive disorder is among the most prevalent and disabling conditions in global medicine, yet its biological underpinnings remain incompletely understood, and current pharmacological treatments fail to produce adequate responses in approximately one-third of patients. A rapidly accumulating body of evidence has not simply challenged the long-dominant monoamine deficiency hypothesis but has provided a mechanistic framework that may explain many of the observed monoaminergic alterations in depressive cohorts, including IDO1-driven serotonin depletion, cytokine-mediated AMPA receptor internalization, and HPA-immune feedback dysregulation. Neuroinflammation, particularly glial activation across microglial, astrocytic, and oligodendrocyte lineages, and its downstream consequences for tryptophan metabolism, glutamatergic transmission, synaptic plasticity, and neurotrophic signaling, represents a central pathophysiological mechanism in a substantial subgroup of depressed patients. Peripheral inflammatory markers, including C-reactive protein, interleukin-6, and tumor necrosis factor-alpha, are elevated in a significant proportion of individuals with major depressive disorder, and these elevations predict poor response to conventional antidepressants while identifying patients who may respond preferentially to anti-inflammatory strategies. Post-mortem studies, positron emission tomography imaging of translocator protein density, and transcriptomic analyses of brain tissue have collectively provided consistent, though not yet fully definitive, evidence that microglial activation is a neurobiological feature of depression rather than a consequence of comorbid physical illness. This review synthesizes current mechanistic understanding of the neuroinflammatory hypothesis of depression, examines the evidence base from epidemiological, biomarker, neuroimaging, and interventional studies, including null findings and methodological limitations, evaluates emerging therapeut