The article discusses the universality of ac conduction in disordered solids, highlighting that despite differences in materials, their ac conductivities follow similar patterns when scaled appropriately. The focus is on the strikingly universal ac properties observed in various solids, such as ion conducting glasses, amorphous semiconductors, and polymers. The key finding is that the ac conductivity of these materials becomes independent of the details of disorder in the extreme disorder limit, where local mobilities vary over many orders of magnitude. This universality is attributed to an underlying percolation process that governs both dc and ac conductivity in such limits. Two models are reviewed: a macroscopic model and a microscopic symmetric hopping model. Both models predict that the normalized ac conductivity, when scaled appropriately, becomes independent of the disorder details. The macroscopic model considers a solid with spatially varying conductivity, while the symmetric hopping model describes charge carriers hopping between localized states. Both models show that the ac conductivity in the extreme disorder limit is governed by percolation, leading to universal behavior. The article also presents analytical approximations and computer simulations that support the universality of ac conductivity. It discusses the implications of these findings for understanding disordered materials and highlights the importance of percolation in explaining the observed universal behavior. The results suggest that the ac conductivity of disordered solids can be described by a universal function, independent of the specific material, and that this universality is a result of the underlying percolation process. The study concludes that the observed ac universality is a consequence of the percolation mechanism, which determines the conductivity in the extreme disorder limit.The article discusses the universality of ac conduction in disordered solids, highlighting that despite differences in materials, their ac conductivities follow similar patterns when scaled appropriately. The focus is on the strikingly universal ac properties observed in various solids, such as ion conducting glasses, amorphous semiconductors, and polymers. The key finding is that the ac conductivity of these materials becomes independent of the details of disorder in the extreme disorder limit, where local mobilities vary over many orders of magnitude. This universality is attributed to an underlying percolation process that governs both dc and ac conductivity in such limits. Two models are reviewed: a macroscopic model and a microscopic symmetric hopping model. Both models predict that the normalized ac conductivity, when scaled appropriately, becomes independent of the disorder details. The macroscopic model considers a solid with spatially varying conductivity, while the symmetric hopping model describes charge carriers hopping between localized states. Both models show that the ac conductivity in the extreme disorder limit is governed by percolation, leading to universal behavior. The article also presents analytical approximations and computer simulations that support the universality of ac conductivity. It discusses the implications of these findings for understanding disordered materials and highlights the importance of percolation in explaining the observed universal behavior. The results suggest that the ac conductivity of disordered solids can be described by a universal function, independent of the specific material, and that this universality is a result of the underlying percolation process. The study concludes that the observed ac universality is a consequence of the percolation mechanism, which determines the conductivity in the extreme disorder limit.