The paper discusses the universality of ac conduction in disordered solids, focusing on experimental results, modeling, and computer simulations. It highlights that various disordered solids exhibit similar ac conductivities, which are independent of the details of the disorder when the local mobilities cover many orders of magnitude, approaching the extreme disorder limit. The authors review two models: a macroscopic model and a microscopic symmetric hopping model. Both models predict ac universality, which is attributed to underlying percolation mechanisms. The paper presents analytical approximations to the universal ac conductivities and compares them with computer simulations. It also discusses the implications of ac universality and its physical causes, emphasizing the role of percolation in determining conductivity in the extreme disorder limit.The paper discusses the universality of ac conduction in disordered solids, focusing on experimental results, modeling, and computer simulations. It highlights that various disordered solids exhibit similar ac conductivities, which are independent of the details of the disorder when the local mobilities cover many orders of magnitude, approaching the extreme disorder limit. The authors review two models: a macroscopic model and a microscopic symmetric hopping model. Both models predict ac universality, which is attributed to underlying percolation mechanisms. The paper presents analytical approximations to the universal ac conductivities and compares them with computer simulations. It also discusses the implications of ac universality and its physical causes, emphasizing the role of percolation in determining conductivity in the extreme disorder limit.