In this talk, I plan to discuss how differential geometry can provide useful insights into the study of ordinary and partial differential equations. In particular, I will focus on the role of symplectic geometry in classical Lagrangian and Hamiltonian mechanics, as well as its generalization to the multisymplectic geometry of classical field theory. Finally, I will talk about how this perspective has paved the way for the development of ``geometric'' numerical integrators, which exactly preserve important structures, symmetries, and invariants.