The critical thickness for (001) CdTe grown on Cd 1−xZn xTe is given for x ranging from 0.03 to 1, i.e. a lattice mismatch from 2×10 −3 to 6×10 −2. Keeping this in mind we can grow coherent heterostructures with good optical properties. In the case of (001) growth the 2s exciton is observed in CdTe-ZnTe quantum wells. For (111) growth the local strain gives rise to a strong piezoelectric field which dramatically decreases the energy of the transitions down to below the CdTe gap and even across the GaAs gap.