Abstract. We consider a dynamical system consisting of subsystems indexed by a lattice. Each subsystem has one conserved degree of freedom ("energy") the rest being uniformly hyperbolic. The subsystems are weakly coupled together so that the sum of the subsystem energies remains conserved. We prove that the subsystem energies satisfy the diffusion equation in a suitable scaling limit.1. Coupled Maps with a Conservation Law 1.1. Diffusion in Hamiltonian Dynamics. One of the fundamental problems in deterministic dynamics is to understand the microscopic origin of diffusion. On a microscopic level, a physical system such as a fluid or a crystal can be modeled by Schrödinger or Hamiltonian dynamics, with a macroscopic number of degrees of freedom. Although the microscopic dynamics is not dissipative, dissipation should emerge in large spatial and temporal scales e.g. in the form of diffusion of heat or of concentration of particles.Dynamically, diffusion is related to the existence in the system of conserved quantities such as the energy which are extensive i.e. sums (or integrals) of local contributions that are 'almost conserved". Thus, if the system has a microscopic energy density E(t, x), x ∈ R d the total energy E tot = E(t, x)dx is a constant of motion but the energy density is, in general, not conserved since the dynamics redistributes it:The divergence acting on the energy current J guarantees conservation of the total energy. One would like to show that the conservative dynamics (1.1) turns, in a suitable scaling limit, to a diffusive one. Such a limit involves diffusive scaling of space and time, and taking typical initial conditions with respect to the Liouville measure with prescribed initial energy profile. The resulting macroscopic energy density should then satisfy a nonlinear diffusion equation of the typewhere κ(E) is the conductivity function.There has been a lot of numerical and theoretical work in recent years around these questions in the context of coupled dynamics. One considers a dynamical system consisting of a large number of elementary systems indexed by a subset V of a ddimensional lattice Z d . The total energy E of the system is a sum x∈V E(x) of energies E(x) which involve the dynamical variables of the system at lattice site x and