The effects of lattice relaxation in thin samples of compositionally modulated semiconductors are discussed. It is shown that even for very weak strain modulations ( less than 10** minus **3) strong diffraction contrast can occur. This is the origin of some 'tweed'-like contrast seen in quaternary semiconductor alloys. Relaxation can also affect the lattice parameter in strained-layer superlattices and can explain some anomalous reports. Finally, effects on high-resolution imaging of interfaces between strained-layers are considered.