  We present low-energy theorems for the calculation of loop amplitudes with
external scalar or pseudoscalar Higgs bosons which are light compared to the
loop particles. Starting from existing lowest-order versions of these theorems,
we show how their applicability may be extended to the two--loop level. To
illustrate the usefulness of these theorems, we discuss a number of
applications to Higgs production and decay at and beyond the one--loop order.
