Recent developments in nonlinear dynamics has significantly altered our basic understanding of the foundations of classical physics. However, it is quantum mechanics, not classical mechanics, which describes the motion of the nucleons, atoms, and molecules in the microscopic world. What are then the quantum signatures of the ubiquitous chaotic behavior observed in classical physics? In answering this question one cannot avoid probing the deepest foundations connecting classical and quantum mechanics. This monograph reviews some of the most current thinkings and developments in this exciting field of physics.