The mechanical and strength properties of paper reflect the intrinsic chemistry, morphology, and structure of the individual fibers as well as the network structure of the paper. These properties also reflect those chemical changes that cause the paper to lose its permanence with time. Strength properties can thereby serve as indicators of the permanence of paper, even when the nature of the chemical changes responsible for the deterioration remains unknown. Careful monitoring of strength properties can also be used for evaluating the effectiveness of treatments for aging. This requires test methods and procedures that have been proven to be reliable. TAPPI test methods for evaluating the properties of tensile strength, bursting strength, tearing resistance, and folding endurance are discussed in terms of the fundamental characteristics of paper on which they depend. Also discussed is the concept of strength-property monitoring in conjunction with accelerated-aging studies. Environmental conditions, primarily relative humidity, can have a great effect on measured strength properties. Testing methods are being developed that employ a greater degree of control over environmental conditions and rates of loading. The fundamental mechanical properties of paper revealed by these methods promise a more complete description of the behavior of paper in environments of changing relative humidities.