Many-body chaos is an important characterization of dynamical systems in the approach to thermalization. In the first part of the talk I shall discuss how chaos sets in an interacting many-body classical system at finite temperature. Using a suitable ensemble quantified by defect density and strength, I shall discuss how a single defect in the symmetry broken phase at zero temperature gives rise to a secondary light cone inside the primary one. This secondary lightcone emerges from the spin waves of the defect, and in the long time limit, as I shall discuss, multiple scattering of many such secondary fronts due to finite defect density gives rise to the onset of chaos.

In the second part, I shall discuss how quantum evolution under continuous weak measurements can be described by a Langevin equation in the semiclassical limit. Using Keldysh field theory, I shall show how the dissipation with an effective bath can be related with the measurement strength in the parent quantum model. By changing the dissipation strength, I shall further show numerically that the chaotic to non-chaotic transition under the dissipative Langevin dynamics can be related to some sort of measurement-induced transition of the original quantum model.