The effects of wall blowing or suction on the stability characteristics of a laminar incompressible two-dimensional plane wall jet are investigated both experimentally and theoretically. A quantitative comparison between linear stability calculations and phase-locked experimental data, obtained when the wall jet is subjected to two-dimensional excitations, confirms the co-existence of the viscous and inviscid instability modes and the theoretically predicted effects of blowing and suction on the stability of the wall jet. According to these predicted effects, blowing stabilizes the inviscid mode while destabilizing the viscous one; suction has the opposite effect. Furthermore, blowing and suction tend to increase and decrease, respectively, the ratio between the outer and inner amplitude maxima of the streamwise velocity fluctuation. When wall blowing is applied, the instability domain is enlarged and includes higher-frequency waves. In addition, the region where both unstable modes co-exist simultaneously begins at a lower local Reynolds number. Opposite effects are caused when suction is applied. The quantitative comparison between the theory and experiment includes the cross-stream structure and the downstream growth of the streamwise velocity fluctuations. In order to accurately account for the effect of the mean flow divergence in the stability analysis, the second-order corrections to the mean flow solutions are obtained for all wall conditions. Spectral distributions, obtained when natural wall-jets are subjected to blowing and suction, support qualitatively the above results.