Simple-minded systems in stable module categories are defined by
orthogonality and generating properties so that the images of the simple
modules under a stable equivalence form such a system. Simple-minded systems
are shown to be invariant under stable equivalences; thus the set of all
simple-minded systems is an invariant of a stable module category. The
simple-minded systems of several classes of algebras are described and
connections to the Auslander-Reiten conjecture are pointed out.