Differential geometry techniques have very useful and important applications in partial differential equations and quantum mechanics. This work presents a purely geometric treatment of problems in physics involving quantum harmonic oscillators, quartic oscillators, minimal surfaces, Schrödinger's, Einstein's and Newton's equations, and others. Historically, problems in these areas were approached using the Fourier transform or path integrals, although in some cases, e.g., the case of quartic oscillators, these methods do not work. New geometric methods, which have the advantage of providing quantitative or at least qualitative descriptions of operators, many of which cannot be treated by other methods, are introduced. And, conservation laws of the Euler--Lagrange equations are employed to solve the equations of motion qualitatively when quantitative analysis is not possible. Main topics include: Lagrangian formalism on Riemannian manifolds; energy momentum tensor and conservation laws; Hamiltonian formalism; Hamilton--Jacobi theory; harmonic functions, maps, and geodesics; fundamental solutions for heat operators with potential; and variational approach to mechanical curves. The text is enriched with good examples and exercises at the end of every chapter.