Gamma‐ray wavelengths measured with the joint NIST/ILL GAMS4 facility at the High Flux Reactor, Grenoble, France, are discussed. This primary goal of these measurements is gamma‐ray wavelengths which are consistent with the optical wavelength scale and the Rydberg constant with an uncertainty no larger than 0.1 ppm for energies up to 5 MeV. The current status of the Bragg angle and crystal lattice spacing measurements on reference energy values, the neutron mass, and the determination of fundamental constants is reviewed. Measurement of structure factors at high energies is also considered.