Local quantum field theories can be defined on any manifold. Often, the spacetime of interest includes a copy of R , like R d (flat space), R × × S d − − 1 (an infinite hollow cylinder), R × × T d − − 1 (space is taken to be a torus) or even Anti-de Sitter space in global coordinates. On such a manifold we can take time to run along R , such that energies are conserved. Solving such a QFT amounts to finding the spectrum and eigenstates of the Hamiltonian H, which is difficult or impossible to do analytically. Hamiltonian truncation provides a strategy to compute the spectrum of H to arbitrary precision. The idea is that many QFT Hamiltonians can be written as the sum of "free" part H 0 and an "interacting" part that describes interactions (for example a ϕ ϕ 4 term or a Yukawa coupling), schematically