The content of the volume is constituted by four articles. The first concerns the theory of propagation of plane waves in elastic media. The second treats theoretically the linear, weakly non-linear, and non-linear stability of flows of a viscous incompressible fluid in a diverging channel. The third lecture investigates the mathematical properties of the equations governing the motion of a viscous incompressible second-grade fluid, such as existence, uniqueness of classical solutions and stability of steady-state flows. The last lecture provides some basic results on wave propagation in continuum models. The objective of this book is to emphasize and to compare the various aspects of interest which include the necessary mathematical background, constitutive theories for material of differential type, polarized and shock waves, and second sound in solids at low temperatures.