The Principle of Least Action is both profound and practical. Since its first formulation by Maupertuis and Euler nearly three centuries ago, the Principle has been, and continues to be, a formidable battlehorse for penetrating unchartered territory in

theoretical physics. The Principle, its connection with, and implications for, our ideas of symmetry, space, time, quantum mechanics, thermodynamics and gravitation, are glanced at.