In this thesis, the main focus is on the interaction of an incoming Tollmien–Schlichting wave with an isolated, stationary wall roughness in subsonic flow. In Part I, this problem is analysed by means of the Triple Deck theory. The linearised sublayer equations are solved under the assumption that the horizontal extent of the roughness is of O(L Re^(−3/8)) and that its height h is small, and an expression for the pressure perturbation is found. The transmission coefficient T_I , defined as the amplitude of the T–S wave downstream of the roughness divided by its initial amplitude, is then calculated, where |T_I | > 1 means that the wave is amplified and |T_I | < 1 represents an attenuation of the T–S wave. The transmission coefficient is dependent on the frequency ω, the height h of the roughness and on the Fourier transform of the roughness shape evaluated at zero value of the wavenumber. The same setup is investigated in Part II through numerical calculations: a DNS solver provides the base flows for 25 different gaps of varying width and height, which are then run through a PSE analysis to examine the stability of the flow. From the results of both methods it can be concluded that a surface indentation amplifies an incoming T–S wave, and that the amplification increases with the width and depth of the roughness. An additional geometry is studied in Part I by again employing the Triple Deck theory to investigate the effect small elastic vibrations of a semi-infinite plate attached to a stationary plate have on the boundary layer, and the receptivity coefficient is calculated for varying ω.