Hodge's index theorem: The index (signature) $\sigma(M)$ of a compact Kähler manifold $M$ of complex dimension $2n$ can be computed by the formula $$\sigma(M) = \sum_{p,q\,:\,p+q\,\text{even}} (-1)^{p+q} h^{p,q}$$ where $h^{p,q} = \dim H^{p,q}(M)$ is the dimension of the space of harmonic forms of type $(p,q)$ on $M$ (cf. Harmonic form). This was proved by W.V.D. Hodge .
Hodge's theorem on the decomposition of the space of smooth sections of an elliptic complex on a compact manifold into the orthogonal direct sum of subspaces of harmonic exact and co-exact sections (see Laplace operator). This was proved by W.V.D. Hodge  for the de Rham complex $$E^*(M) = \sum_{p\ge0} E^p(M)$$ on an orientable compact Riemannian manifold $M$. In this case Hodge's theorem asserts that for any $p\ge0$ the space $H^p(M)$ of harmonic forms on $M$ is finite-dimensional and that there exists a unique operator $G : E^p(M) \rightarrow E^p(M)$ (the Green–de Rham operator) satisfying the conditions $$G(H^p(M)) = 0 \ ;\ \ \ Gd = dg\ ;\ \ \ G \delta = \delta G$$ $$E^p(M) = H^p(M) \oplus d \delta GE^p(M) \oplus \delta d G E^p(M)$$ (the Hodge decomposition). In particular, $H^p(M)$ is isomorphic to the real cohomology space $H^p(M,\mathbf{R})$ of $M$. Another important special case is the Hodge theorem for the Dolbeault complex on a compact complex manifold $M$ (see Differential form) . These results lead to the classical Hodge structure in the cohomology spaces of a compact Kähler manifold.