In this thesis, we study the effectiveness of compliant boundaries on the absolute instability (AI) ofconfined 2D inviscid jets and wakes, and compare the results with the effects of rigid impermeableboundaries. We consider symmetric and asymmetric configurations. We define AI in the long timelimit after an impulsive perturbation has been made to the flow. If we see growth in time as wellas in space, both upstream and downstream of the initial perturbation than this constitutes an AI.The characteristics of the AI is quantified by the sign and magnitude of the imaginary part of thetemporal frequency ω.We construct the governing boundary conditions for the compliant walls, and derive analytic dis-persion relations D(α, ω) = 0 for the flow for both varicose and sinuous modes in the case ofsymmetric flows. Furthermore, we initially consider the limit of zero shear layer thickness but go onto consider more realistic flow configurations with finite thickness shear layers and smooth velocityprofiles. We plot contours of constant ωi into the complex α-plane through the dispersion relationto locate modes of instability, which take the form of saddle points. Using Brigg’s Criterion, we de-termine which saddle points contribute to the AI characteristics of the flow, known as pinch points.We use numerical techniques to determine the precise values of α and ω associated with each pinchpoint, and to explore how these values evolve as we modify flow or wall parameters.It is shown that compliant walls modify existing shear-induced modes (SI), while also inducing newwall-induced (WI) modes. These new modes are able to dominate the flow’s AI response. Byasymmetrically confining the flow, we find that the location of the closest bounding wall is usuallykey in determining the flow’s instability characteristics. When the wall is placed in an optimumposition, WI modes are able to persist with even larger growth rates even when one of the twowalls is taken away. This behaviour is also seen when the walls are rigid, but the mechanism behindthis behaviour is not driven by wall compliance in this case. By making the walls non-identical inconstruction, on the other hand, we observe a family of AI modes associated with each wall. Whenone wall is taken to be rigid, the WI modes associated with the wall stabilize, while the modesassociated with the remaining compliant wall remain unstable, suggesting that only one compliantwall is sufficient to generate strong WI modes.Compliant boundaries can readily influence the flow’s stability characteristics, both in a symmetricand asymmetric configuration. This has practical applications in engineering for use in cases whichcould benefit from modifying the flow’s instability, such as improving the mixing between shearlayers, or as a means of reducing sound pollution.