We discuss the effects of instantons in partially broken gauge groups
on the low-energy effective gauge theory. Such effects arise when some
of the instantons of the original gauge group G are no longer contain
ed in (or can not be gauge rotated into) the unbroken group H, In case
s of simple G and H, a good indicator for the existence of such instan
tons is the ''index of embedding.'' However, in the general case one h
as to examine pi(3)(G/H) to decide whether there are any instantons in
the broken part of the gauge group. We give several examples of super
symmetric theories where such instantons exist and leave their effects
on the low-energy effective theory, (C) 1998 Elsevier Science B.V.