Frustration is present in most antiferromagnetic compounds, either due
to the lattice structure or to the existence of competing long-range
interactions. We show that, close to a magnetic-nonmagnetic instabilit
y, frustration can lead to the stabilization of phases where both magn
etic and nonmagnetic sites are present. Several examples are discussed
: the RMn2 compounds where the Mn 3d magnetic moment is unstable; CeSb
and UNi4B in which the Kondo effect can be responsible for the instab
ility. We present a model which includes both ingredients: frustration
and magnetic instability, and explain qualitatively the unusual magne
tic structures observed in these compounds.