Generalized-ray theory for time-domain electromagnetic fields in a horizontally layered medium is developed. After introducing appropriate integral transformations and source-type field representations in vertically inhomogeneous media, the solution is written out in terms of generalized ray constituents whose space-time counterparts are constructed with the aid of the Cagniard-DeHoop technique. The formulation lays the foundation to rigorously study time domain field behavior in numerous practical topologies where a stratified multilayer is involved, such as planar antennas and circuits, but also EMC and propagation problems.