The energy release rate G(s), function of the arc length s along the front ∂Γ of a crack Γ: ∫∂ΓG(s)δl(s)ds=−δW where δW is the perturbation of the elastic potential energy at equilibrium W induced by a crack front normal extension δℓ, the load being kept fixed, is one of the basic quantities involved in elastic fracture mechanics. In linear fracture mechanics, G is linked to the stress intensity factors KI(s), KII(s), KIII(s) through Irwin formula: G(s)=1+v2μ[K2I(s)+K2II(s)]+12μKIII(S) (μ: shear modulus, v: Poisson ratio). In addition, G has a clear thermodynamical meaning and plays a key role in Griffith-type crack extension criteria.