This short review presents the author’s studies on the failure behavior of polymers and polymer-based composites. The main contents are as follows; 1) macroscopic criteria for failure in polymeric glasses, 2) environmental stress cracking (ESC) in polymeric glasses, 3) self-healing in poly (methyl methacrylate) induced by case II diffusion of methanol, 4) detection of micro-defects in plastically deformed polyamide 6 (PA6) films by means of thermoporosimetry on absorbed water, 5) dynamic viscoelastic analyses of fatigue behavior of polymer blends of polypropylene and liquid crystalline polyester (PP/LCP), 6) time-water content superposition in dynamic viscoelasticity of PA6-carbon fiber (CF) model composites, 7) interfacial degradation in model composite films of unsaturated polyester and aramid fiber analyzed by micro-Raman and infrared spectroscopy, 8) evaluation of interfacial microstructure of PA6-CF model composite films by differential scanning calorimetry (DSC) on freezing behavior of absorbed water.