The Hilbert scheme X [a] of points on a complex manifold X is a compactification of the configuration space of a-element subsets of X . The integral cohomology of X [a] is more subtle than the rational cohomology. In this paper, we compute the mod 2 cohomology of X [2] for any complex manifold X , and the integral cohomology of X [2] when X has torsion-free cohomology.2010 Mathematics Subject Classification: 14C05 (primary); 55R80 (secondary)For a complex manifold X and a natural number a, the Hilbert scheme X [a] (also called the Douady space) is the space of 0-dimensional subschemes of degree a in X . It is a compactification of the configuration space B(X, a) of a-element subsets of X . The Hilbert scheme is smooth if and only if X has dimension at most 2 or a 3 [3, equation (0.2.1)]. The integral cohomology of the Hilbert scheme is more subtle than the rational cohomology. Markman computed the integral cohomology of the Hilbert schemes X [a] for X of dimension 2 with effective anticanonical divisor [10]. In this paper, we compute the mod 2 cohomology of X [2] for any complex manifold X , and the integral cohomology of X [2] when X has torsion-free cohomology. In one way, things are unexpectedly good: the Hilbert scheme X [2] has torsionfree cohomology if X does (Theorem 2.2). On the other hand, the details are intricate, and it was not clear that complete answers would be possible. The behavior of the inclusion of the exceptional divisor E X into X [2] is related to the Steenrod operations on the mod 2 cohomology of X (Theorem 2.1). To explain one difficulty: some cohomology classes on X [2] can be defined as the classes