(First of all apologies for the long wall of text) I am to study BRST transformations, for which I'm currently trying to understand constrained Hamiltonian dynamics to treat systems with singular Lagrangians. The crude recipe followed is Lagrangian -> Hamiltonian -> Dirac brackets and their quantization. I have been told that these techniques are used in QFT, string theory/high energy , etc. My question is are these formalisms indeed good and useful to learn? I'm confused because there are other formalisms and recipes (for example directly working with the Lagrangian). From a modern perspective, are these relevant? And does there exist any other formalism that I should not ignore? I will be starting my graduation soon; is it late to be studying constrained H formalism/BRST? (I want to know how beneficial it is to study constrained Hamiltonian approach, because I'm not sure whether it's obsolete, having been replaced by better approaches (like path integrals?) which I should rather pay attention to. I'm not even sure what is the extent of applicability of constrained Hamiltonian dynamics..) Thanks EDIT: Directly working with Lagrangian as in one doesn't bother with the Hamiltonian in that approach, direct quantization from the Lagrangian stage. I have also heard that there are other approaches like Feynman path integrals, etc. But I don't know about their relative merits, and that's what I want to be clear about.