This chapter considers internal as well as input-output stability of closed-loop MIMO systems. For internal stability, an important observation is that four sensitivity transfer functions relating exogenous to internal signals must be proper and stable, the “Gang of Four”, as Åström has termed them. If one of the sensitivity-related transfer functions is not stable, then the closed-loop feedback system is not internally stable. For input-output stability of MIMO feedback systems, the chapter presents the generalised Nyquist criterion, the use of eigenvalue loci and the small-gain theorem. The latter one is particularly useful for the stability analysis of plants subject to perturbations. With the analysis of stability, two questions naturally come up: What about the stability of a MIMO feedback loop if perturbations are present? When is a closed feedback loop stable with a MIMO plant and a decentralised controller? The first question is addressed in Chap. 11 on robust control. The answer to the second question depends on whether the return difference matrix is diagonal dominant.