Probing Exotic Higgs Sectors in $\ell^-\ell^-$ Collisions

I review extended Higgs sectors and constraints thereon arising from $\rho=1$, gauge-coupling unification and $b\to s\gam$. 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.