Dynamical dimensional reduction or spontaneous dimensional reduction is the apparent reduction in the number of spacetime dimensions as a function of the distance scale, or conversely the energy scale, with which spacetime is probed. At least within the current level of experimental precision, our universe has three dimensions of space and one of time. However, the idea that the number of dimensions may increase at extremely small length scales was first proposed more than a century ago, and is now fairly commonplace in theoretical physics. Contrary to this, a number of recent results in quantum gravity suggest the opposite behavior, a dynamical reduction of the number of spacetime dimensions at small length scales.