In the spectroscopy of atoms and molecules, an asymmetric Fano resonance arises whenever a bound state associated with one electronic configuration is coupled to the ionization continuum of a different configuration. A strikingly similar resonance appears for electronic transport in conductors with more than one subband, independent of the specific details of the system under study. We develop a two-subband approximation which describes the Fano resonances for conduction through an electron waveguide containing donor impurities, for Γ-X-Γ intervalley tunneling in a GaAsAlxGa1-xAsGaAs heterojunction, and for an electron waveguide coupled to a resonant cavity. Interference between the direct and intersubband transmission channels gives rise to the asymmetric Fano resonance. © 1993 The American Physical Society.