The Hilbert scheme is a classically studied moduli space, which parametrises closed subschemes with prescribed Hilbert polynomial in an ambient projective scheme. These moduli spaces where initially introduced by Grothendieck, but turned out to have a prominent rôle in many modern areas outside the realm of Algebraic Geometry: Mathematical Physics, Combinatorics, Theoretical Physics (just to mention some recent examples). Generalisations of the Hilbert scheme have a wide range of applications as well, such as nested Hilbert schemes (parametrising flags of closed subschemes), moduli spaces of framed sheaves and Quot schemes (parametrising quotient sheaves).

The easiest example one can consider is the Hilbert scheme of points, which parametrises closed zero-dimensional subschemes of fixed length. Already in this case, the geometry of the moduli space is rich and intriguing, as it is in general considerably pathological. For instance, if the dimension of the ambient variety is 3, the Hilbert scheme of points is in general reducible, and for dimension larger than 4 its singularities can be as bad as possible.

Hilbert schemes (and their generalisation) played an important role in the development of modern Enumerative Geometry. In fact, for a large and interesting class of cases, the Hilbert scheme of points is not smooth but carries a virtual fundamental class. Donaldson-Thomas theory deals precisely with the (virtual) invariants one can compute on the Hilbert schemes with respect to its virtual structure. Donaldson-Thomas theory is very rich and admits several layers of refinements, for example: cohomological, K-theoretical, elliptic, categorical, motivic and is predicted to match Gromov-Witten invariants.