Hamiltonian truncation is a numerical method used to study quantum field theories (QFTs) in d ≥ ≥ 2 spacetime dimensions. Hamiltonian truncation is an adaptation of the Rayleigh–Ritz method from quantum mechanics. It is closely related to the exact diagonalization method used to treat spin systems in condensed matter physics. The method is typically used to study QFTs on spacetimes of the form R × × M , specifically to compute the spectrum of the Hamiltonian along R . A key feature of Hamiltonian truncation is that an explicit ultraviolet cutoff Λ Λ is introduced, akin to the lattice spacing a in lattice Monte Carlo methods. Since Hamiltonian truncation is a nonperturbative method, it can be used to study strong-coupling phenomena like spontaneous symmetry breaking.