<p>Proton exchange membrane fuel cells (PEMFCs) are promising clean energy technologies due to their high efficiency, low operating temperature, and environmental compatibility. However, their commercial viability is limited by membrane degradation, which undermines long-term durability. This study reviews the fundamental degradation mechanisms of PEMFC membranes: mechanical stress from hydration cycling, and chemical and thermal stresses; and evaluates accelerated stress testing (AST) as an investigative approach for durability assessment. Standardized U.S. Department of Energy (DOE) chemical and mechanical AST protocols are compared with modified protocols reported in the literature to highlight advancements in test design and execution. Diagnostic tools such as polarization curves, linear sweep voltammetry, cyclic voltammetry, electrochemical impedance spectroscopy, and electrochemical leak detection tests are discussed in relation to hydrogen crossover, membrane thinning, and ionic resistance. Failure criteria, including open-circuit voltage loss, electrochemical leak detection, and high-frequency resistance, are presented to ensure experimental validity. Finally, mitigation strategies such as the use of radical scavengers, reinforced membranes, and chemically stable polymer structures are reviewed to improve membrane durability under accelerated stress conditions.</p>