Materials that exhibit a yield stress are characterized by a tridimensional structure, and can be described by a quantity of n structural points in a volume V. For the material to flow, this number n must be inferior to a critical number n(0). A simple kinetic equation is proposed to describe the evolution of n under the influence of an applied stress or shear. The conditions for flow to occur and the time required to maintain the stress or strain before the material begins to flow are deduced. The response of such a material under an imposed sinusoidal shear is also investigated. It is assumed that when the magnitude of the stress is less than the yield stress the material is Hookean; otherwise the stress is a linear function of shear rate. The storage and dynamic moduli are calculated.