We first sketch the general concepts of the lattice regularisation in quantum field theory, which does not require gauge fixing, and which enables Monte Carlo simulations that yield non-perturbative numerical measurements of observables. We then address the status of lattice QCD with 2+1 flavours of dynamical quarks, where hadron masses can now be evaluated from the first principles of QCD close to the percent level. In a second part we discuss the relation between QCD and Chiral Perturbation Theory, which can be strengthened by the determination of Low Energy Constants based on lattice simulations. In particular we illuminate this procedure by referring to the residual pion mass in the delta-regime. Finally we address the issue of measurements at fixed topology, which is going to be relevant in future simulations, to be performed even closer to the continuum limit.