Structures containing composition modulations (whether produced naturally or artificially) can contain coherency strains of up to several percent. Such strains can normally only be produced by hydrostatic pressures exceeding 10 GPa (100 kb). Expressions are derived for the calculations of these strains, and the effect that they can have on the properties of a modulated structure is discussed. A summary is also given of the studies made at Northwestern University of composition modulated foils produced by vapor deposition. Four of the systems (Au‐Ni, Cu‐Ni, Cu‐Pd, and Ag‐Pd) investigated exhibited an increase of between two‐ and fourfold in the elastic modulus at short wavelengths (<∠3 nm). An anomalous magnetic effect has also been discovered in Cu‐Ni foils.