Topological string theories in dimension less than one are known to be governed by integrable hierarchies. The most famous example is intersection theory on the moduli space of Riemann surfaces, which according to Witten and Kontsevich is governed by the KdV hierarchy. In this talk I will present a general framework to understand the integrability of topological strings, as well as some new results along this direction for topological strings on noncompact Calabi-Yau threefolds.