The aim of this work is to extend and prove the Onsager conjecture for a class of conservation laws that possess generalized entropy. One of the main findings of this work is the "universality" of the Onsager exponent, α > 1/3, concerning the regularity of the solutions, say in C 0,α , that guarantees the conservation of the generalized entropy; regardless of the structure of the genuine nonlinearity in the underlying system.In this work we aim at extending and proving the Onsager conjecture for a class of conservation laws that admit a generalized entropy. Roughly speaking, the Onsager conjecture [18] states that weak solutions of the three-dimensional Euler equations of inviscid incompressible flows conserve energy if the velocity field u ∈ C 0,α , for α > 1 3 , and that the critical exponent α = 1 3 is sharp. This conjecture has been the subject of intensive investigation for the last two decades. The sufficient condition direction was proved by Eyink [14] for the case when α > 1 2 . Later, a complete proof was established by Constantin, E and Titi [9] (see also [8]) under slightly weaker regularity assumptions on the solution which involve a similar exponent α > 1 3 . Duchon and Robert [13] have shown, under similar sufficient conditions to those in [9], a local version of the conservation of energy. It is worth mentioning that the above results are established in the absence of physical boundaries, i.e., periodic boundary conditions or the whole space. However, due to the well recognized dominant role of the boundary in the generation of turbulence (cf.[4] and references therein) it seems very reasonable to investigate the analogue of the Onsager conjecture in bounded domains. Indeed, for the three-dimensional Euler equations in a smooth bounded domain Ω, subject to no-normal flow (slip) boundary conditions, it has been shown in [5] that a weak solution conserves the energy provided the velocity field u ∈ C 0,α (Ω) , for α > 1 3 , (see also [19] for the case of the upper-half space under stronger conditions on the pressure term). A local version, analogue to that of [13], was established recently in [6] under slightly weaker conditions to those in [5], but at the expense of additional sufficient conditions concerning the vanishing behavior of the energy flux near the boundary.Showing the sharpness of the exponent α = 1 3 in Onsager's conjecture turns out to be much more subtle. This direction has been underlined by a series of contributions (cf. Isett [17], Buckmaster, De Lellis , Székelyhidi and Vicol [7] and references therein) where weak solutions, u ∈ C 0,α , with α < 1 3 , that dissipate energy were constructed using the convex integration machinery. Notice, however, that there exists a family of weak solutions to the three-dimensional Euler equations, that are not more regular than L 2 , and which conserve the energy, cf. [3].It is most natural to ask whether the analogue of the Onsager conjecture is valid for other systems of conservation laws. Indeed, there has been some intensive recent work extendi...