This thesis explores the causes of tooth discoloration and evaluates in-office bleaching as a fast, effective treatment in aesthetic dentistry. It distinguishes between intrinsic and extrinsic stains and emphasizes accurate diagnosis to guide proper technique selection. The chemical mechanisms of hydrogen peroxide and related agents are reviewed, along with procedures for treating both vital and non-vital teeth. Common side effects like sensitivity and gingival irritation are addressed, with strategies for risk reduction. The study highlights both the clinical effectiveness and the psychological benefits of professional tooth whitening.