Abstract.A set E in a space X is called a polar set in X, relative to a kernel k(x, y), if there is a nonnegative measure σ in X such that the potential U σ k (x) = ∞ precisely when x ∈ E. Polar sets have been characterized in various classical cases as G δ -sets (countable intersections of open sets) with capacity zero. We characterize polar sets in a homogeneous space (X, d, µ) for several classes of kernels k(x, y), among them the Riesz α-kernels and logarithmic Riesz kernels. The later case seems to be new even in R n .