We consider two
types of spaces, the Bessel potential spaces Lαp(Rn) and the Besov spaces
Λαp(Rn), α > 0, 1 < p < ∞. Associated in a natural way with these spaces are
classes of exceptional sets. We characterize the exceptional sets for Λαp(Rn) by an
extension property for continuous functions and prove an inequality between Bessel
and Besov capacities.