Some of the dynamic characteristics of the human body were analyzed and mathematical analogs that can be used to predict the response of the body to acceleration environments were developed. The background of the acceleration tolerance problem, the basic concepts of body dynamics, and the available experimental data were also reviewed. Both spinal and transverse dynamic models are presented together with the data used in obtaining frequency, damping, and breaking strength estimates. Other dynamic models are discussed more briefly since very little is known about the dynamic response to lateral and negative spinal accelerations. A discussion of the factors influencing the production of injury from exposure to accelerations is presented to indicate the difficulty in defining human tolerance to acceleration using the classical approach of graphs or simple critical G values that depend only upon the duration of the acceleration. The concept of relative probability of injury is developed in order to take into account the variations in the human structure and other factors that influence human tolerance to accelerations.