Spontaneous and stimulated emission from a series of Al0.7Ga0.3As ‐Al0.49Ga0.51As‐GaAs ultrathin single [1,2, and 3 monolayer (ML), 1 ML=2.83 A˚] and multiple (2 ML) quantum well separate confinement heterostructures are demonstrated and compared to a control sample (0 ML). Spectra from sample to sample are very different and depend on the alignment of then=1, &Ggr; electron bound state in the quantum well with theXminima(lowest band edge) in the indirect‐gap (Al0.49Ga0.51As) confining layers. Some samples (3 ML single and 2 ML multiple quantum well) can support stimulated emission despite the fact that the quantum well is undoped, unstrained, and very thin (Lz≪scattering path length) and that most of the wave function is in the indirect‐gap confining layers. These experimental results can be explained using a simple model based on the spatial extent of the wave function (rather than the well width) under the special condition of band alignment between the &Ggr; andXstates.