  The proper renormalization of mixing angles in quantum field theories is a
long-standing problem. It is relevant for the renormalization of the quark
mixing matrix in the Standard Model and for various mixing scenarios in
theories beyond. In this contribution we specifically consider theories with
extended scalar sectors. We describe renormalization schemes for mixing angles
based on combinations of observables or symmetry requirements such as rigid or
background-field gauge invariance and compare their properties to previous
approaches such as $\bar{MS}$ schemes. We formulate specific renormalization
conditions for the mixing angles in the Two-Higgs-Doublet Model and the
Higgs-Singlet Extension of the Standard Model and calculate electroweak
corrections to Higgs-boson decays via W- or Z-boson pairs within these models
for a selection of (new and old) renormalization schemes.
