This paper summarizes a session presented at the Ninth Annual Frontiers of Science Symposium in 1997. It discusses the development of string theory, which aims to address fundamental questions in physics regarding the nature of particles and the laws governing their behavior. The paper outlines the historical context of particle physics, from molecules and atoms to the standard model, which suggests that there may be more fundamental degrees of freedom. It also reviews the evolution of theories of motion and gravity, leading to general relativity, which faces challenges at very small distance scales where quantum effects become significant. String theory offers a potential solution by modeling particles as one-dimensional strings rather than points, which could resolve the divergence problem in quantum gravity. String theory predicts gravity and includes the graviton, a particle responsible for gravitational interactions. However, our understanding of string theory has been limited to weakly interacting string models, and the fundamental principle underlying the theory remains unknown. The discovery of string duality has been a major breakthrough, revealing equivalences among different physical systems and introducing D-branes. String duality has also led to a revised understanding of space and time, showing that string theories involve extra dimensions, which are compactified to produce the four observable dimensions of our universe. String duality has shown that these compact spaces can be linked together, suggesting that there may be only one possible string-theoretic model of the universe. The paper concludes with a discussion of the rapid development of string theory and the challenge of discovering the underlying principle that may lead to the truly fundamental degrees of freedom in our universe.This paper summarizes a session presented at the Ninth Annual Frontiers of Science Symposium in 1997. It discusses the development of string theory, which aims to address fundamental questions in physics regarding the nature of particles and the laws governing their behavior. The paper outlines the historical context of particle physics, from molecules and atoms to the standard model, which suggests that there may be more fundamental degrees of freedom. It also reviews the evolution of theories of motion and gravity, leading to general relativity, which faces challenges at very small distance scales where quantum effects become significant. String theory offers a potential solution by modeling particles as one-dimensional strings rather than points, which could resolve the divergence problem in quantum gravity. String theory predicts gravity and includes the graviton, a particle responsible for gravitational interactions. However, our understanding of string theory has been limited to weakly interacting string models, and the fundamental principle underlying the theory remains unknown. The discovery of string duality has been a major breakthrough, revealing equivalences among different physical systems and introducing D-branes. String duality has also led to a revised understanding of space and time, showing that string theories involve extra dimensions, which are compactified to produce the four observable dimensions of our universe. String duality has shown that these compact spaces can be linked together, suggesting that there may be only one possible string-theoretic model of the universe. The paper concludes with a discussion of the rapid development of string theory and the challenge of discovering the underlying principle that may lead to the truly fundamental degrees of freedom in our universe.