ZnSe Films and fully developed p-on-n laser structures, including CdZnSe-active and ZnSSe-guiding layers were grown by molecular beam epitaxy (MBE) on lattice matched p-GaAs, p-A1GaAs and p-GaInP buffer layers. The structural characteristics of these layers were studied by combined cross-section and planar view transmission electron microscopy (TEM). The defect density of the ZnSxSe1-x epilayers was shown to be very low, < 10(5) cm(-2). However, on their interfaces with the GaAs substrate, a high density of small dislocation loops and clusters of the order of 3 x 10(10) cm(-2) was observed. In situ TEM experiments revealed that dislocations and stacking faults (SFs) were generated under the electron beam influence. From them, the perfect dislocations were confined at the ZnSe/GaAs interface, while the SFs propagated to the ZnSe overgrowth or the GaAs substrate, having one of their partial dislocations at the interface. The generation of dislocations under the electron beam was not related to radiation damage but to thermal strain, which was developed by the heating effect due to differential thermal dilatation. Defects around the active zone of fully developed p-on-n laser structures were also studied. The nature of such defects was defined by Burgers vector determination experiments. The critical role of growth variations, such as compositional changes resulting in strain, in the MBE process of Il-VI materials was demonstrated. The destructive role of the defected guiding layers in the laser structure was shown.