For the past 30 years, the author has been conducting research on the mechanics of rubber elasticity and fibrous materials. This article presents a historical review of the development of the research. The author’s first research project was an investigation of thickness irregularity in fiber bundles. This irregularity is caused by the random arrangement of the short fibers that form the bundle, and increases during the drafting of the bundle in the spinning process. The author presented a theory that made it possible to predict the power spectrum of this irregularity. This research stimulated the author’s interest in the mechanics of fiber assembly bodies, also known as ‘‘fibrous materials.’’ This interest remains strong today. After completing his first research project, the author spent a year at the California Institute of Technology, in Pasadena, where he initiated research on rubber elasticity as a basis for research on the failure behavior of solid rocket propellant. He has developed a biaxial extension experiment for rubbers, and found a simple form of the strain energy density function for these rubbers. Parallel with this research on rubber elasticity, the author continued research on the deformation mechanism of fibrous materials such as yarns and woven and knitted fabric structures. On the basis of this research, the author developed an objective evaluation of fabric hand; evaluations had previously been made by a subjective method called ‘‘hand judgment.’’ There is a similarity between research on the mechanics of rubber and research on the mechanics of fibrous materials. The research deals with the assembly body of molecular chain or fiber and have been carried out with an interactive concept. The author has also introduced his recent research on the measurement of anisotropy in the mechanical property of single fibers. This measurement was conducted with a micromeasurement instrument developed by the author. Some of the results of these measurements are also presented here.