Naturally occurring ducting environments (for example, tropospheric elevated or surface ducts) may exhibit continuous longitudinal changes that progressively decrease the guiding capability and eventually destroy it altogether. This circumstance has evident implications for systems employing such ducts as communication or surveillance channels. Recent developments in the theory of propagation in longitudinally varying ducts are reviewed, with emphasis on gradual guiding to antiguiding transitions that convert an initially well-trapped mode field smoothly (adiabatically) into a leaky mode. The connection between the trapped and leaky mode regimes is provided by a parabolic equation or, in certain cases, by an “intrinsic mode” spectral integral. Numerical implementation for different examples confirms the phenomenology predicted by the analysis and reveals the beaming of initially trapped modal energy into the exterior after the duct disappears. Some consideration is also given to incorporating these concepts into a synthesis of the field excited by a radiating source inside the duct.