The dynamics of the fluctuating velocity field in a turbulent boundary layer is discussed on the basis of a simplified analysis of localized unsteady perturbations in a parallel shear flow. Three classes of such disturbances may be distinguished, namely: (i) large‐scale propagating disturbances which may be represented by a superposition of shear waves; (ii) convected disturbances which decay only slowly through the action of viscosity; and (iii) small‐scale disturbances resulting from the secondary instability of the large‐scale motion. Coupling between large and small scales is incorporated in a two‐scale model in which the large scales are considered driven by the Reynolds stresses produced by the small scales. Shearing by the mean flow is shown to cause intensification of internal shear layers in the flow and could lead to local inflection in the instantaneous velocity profile, thus making secondary instability possible. Drag reducing additives, as modeled by some simple constitutive relations are found to be able to inhibit secondary instability, and would hence lower turbulent stress production according to the model employed.