This paper deals with an exact approach to the dynamic stability of orthotropic sheardeformable viscoelastic flat plates subjected to in-plane uni/biaxial edge load systems. In deriving the associated governing equations a Boltzmann hereditary law is used and in addition transverse shear deformation, transverse normal stress and rotatory inertia effects are incorporated. The integro-differential equations governing the stability of simply-supported flat plates are solved in the Laplace transform (LT) space in order to determine the critical in-plane edge loads yielding the asymptotic instability of flat plates. The stability analysis allows one to obtain the nature of the loss of stability i.e. either by divergence or by flutter. Numerical applications are presented and pertinent conclusions are formulated.