Probing Exotic Higgs Sectors in ${\mathrm{\ell }}^{-}{\mathrm{\ell }}^{-}$ Collisions

I review extended Higgs sectors and constraints thereon arising from $\rho =1$, gauge-coupling unification and $b\to s\text{gam}$. The couplings and decays of the Higgs boson eigenstates are outlined for triplet representations. Direct experimental probes of exotic Higgs bosons are reviewed with a focus on the important role that would be played by an $e^{-}{e}^{-}$ or ${\mu }^{-}{\mu }^{-}$ collider.