  Numerical Campedelli surfaces are minimal surfaces of general type with p_g=0
(and so q=0) and K^2=2. Although they have been studied by several authors,
their complete classification is not known. In this paper we classify numerical
Campedelli surfaces with an involution, i.e. an automorphism of order 2. First
we show that an involution on a numerical Campedelli surface S has either four
or six isolated fixed points, and the bicanonical map of S is composed with the
involution if and only if the involution has six isolated fixed points. Then we
study in detail each of the possible cases, describing also several examples.
