This book addresses a core gap in traditional physics education by exploring the Lagrangian formalism and its profound applications across several domains. Its primary aim is to answer the persistent question: why is the classical Lagrangian defined as L=T-V, a concept often assumed without explanation in standard undergraduate texts.We employ a coordinate transformation-driven approach, treating coordinate transformations as central to how physical laws are constructed, interpreted, and applied. This methodology allows for a clear derivation of the classical Lagrangian and highlights the invariance of the Euler-Lagrange equation under arbitrary coordinate transformations, which serves as the main motivation for our exploration.While beginning with classical mechanics, the scope of our book naturally extends to unify concepts across a wide range of theoretical physics, including field theories, electrodynamics, and relativity. The text uses the Lagrangian's unique properties to provide an exact mathematical definition for the principle of relativity; establishing what it means for physical laws to be "the same" in different coordinate systems. We also introduce advanced topics, such as the formulation necessary for General Relativity and the Einstein-Hilbert Lagrangian. The book is designed to provide undergraduate accessibility while guiding readers through the deep structural symmetries of physical laws.