This chapter provides a comprehensive analysis of red oral mucosal lesions, examining their aetiology, clinical presentation, diagnostic approaches, and management strategies. It begins by exploring the pathophysiological mechanisms underlying mucosal erythema, including inflammation, infections, trauma, autoimmune responses, and vascular abnormalities. The chapter systematically classifies common red lesions (erythroplakia, erosive lichen planus, erythematous candidiasis, and vascular anomalies), emphasizing their distinguishing clinical and histopathological features. A robust differential diagnosis framework integrates clinical examination, adjunctive tools (toluidine blue staining, brush biopsy), and histopathological evaluation, with case studies illustrating diagnostic challenges. Special attention is given to high-risk lesions like erythroplakia, highlighting their malignant potential. The chapter concludes with emerging technologies (molecular diagnostics, AI-assisted imaging) that are reshaping diagnostic paradigms. Throughout, the focus remains on clinical relevance, particularly early detection of premalignant/malignant conditions and evidence-based management.