Transmission electron microscopy is used to investigate the structure of the crystalline defects in degraded and undegraded ZnSe-based films/GaAs heterostructures. In degraded areas, dark line defects (DLDs) originate from the preexisting or grown-in defects (i.e., stacking faults and misfit dislocations) during device operation and make the laser diode fail. From our observation, the nucleation of DLDs is based on the initial emission of a mobile faulted defect from the preexisting grown-in defects. Along the trace of the faulted defects, [100] dark line defects form. We also investigated the origins and mechanisms for the generation of stacking faults and misfit dislocations. The generation of stacking faults is strongly related to the doping concentration, substrate surface stoichiometry, and growth mode of the films. The vacancy contained Ga-Se interfacial layers are thought to be sources for the generation of Frank-type stacking faults. In addition, Shockley partial dislocations form due to island coalescence of the ZnSxSe1-x/ZnSe epilayers on GaAs substrates. In situ electron beam heating studies were carried out to realize the mechanism of strain relaxation and formation of 60 degrees-type misfit dislocations in the II-VI/GaAs interface from the dissociation of Frank-type partial dislocations. The generation of screw-type interfacial dislocations was also observed. This takes place by gliding the threading segments of the Shockley partial dislocations toward the ZnSxSe1-x/GaAs interface on {111}-type planes.