of the MSSM with complex parameters. In this case ${\mathcal{CP}}$-violating effects entering at the loop level give rise to a mixing between the lowest-order mass eigenstates $h, H, A$, which are ${\mathcal{CP}}$-eigenstates ($h, H$ are ${\mathcal{CP}}$-even states, while $A$ is a ${\mathcal{CP}}$-odd state). The $(3 \times 3)$ mixing between the neutral Higgs states is the smallest type of propagator mixing that exhibits the qualitative features of general $(n \times n)$ propagator matrices. Furthermore, the MSSM example is well-suited to emphasise the distinction between interaction eigenstates (in this case the component field of the two Higgs doublets, e.g. $\phi_1$ and $\phi_2$), lowest-order mass eigenstates ($h, H, A$) and loop-corrected mass eigenstates, which in the MSSM case are called $h_1, h_2, h_3$. The MSSM case also illustrates the possibility of mixing with other spin states ($\gamma, Z$) and with unphysical degrees of freedom ($G$). An important feature of models like the MSSM is the fact that the masses are [*predicted*]{} from the input parameters of the model, which makes the need for an appropriate assignment of mass eigenstates with propagator poles particularly apparent. The generalisation of our results to other models, cases with $(n \times n)$ mixing for $n > 3$ and 