Nonlinear x-ray spectroscopy The extension of nonlinear optics to the x-ray spectral domain is a promising direction in the development of x-ray spectroscopy. Although theoretical concepts of nonlinear x-ray spectroscopy were developed decades ago, scientists still struggle to implement them because of the elusive nature of nonlinear effects. Eichmann et al. now present atomic momentum spectroscopy (AMS), which is based on the detection of the scattered atom after momentum transfer from x-ray photons (see the Perspective by Pfeifer). The authors show how AMS can observe stimulated x-ray Raman scattering signals at the neon K edge on a single-atom level and distinguish them from other competing processes. These results pave the way for future nonlinear x-ray spectroscopy methods for the study of x-ray–matter interactions. Science , this issue p. 1630 ; see also p. 1568