SiGe is of interest for use in heterojunction-bipolar transistors, infrared detectors and field-effect transistors. In this study, graded SiGe heterolayers grown on Si, and heterolayers grown on SIMOX by CVD, were characterized using TEM. The graded-heterolayers consisted of ten layers of Si1-xGex on substrate silicon. Misfit dislocations were present at interfaces in the bottom 4–5 layers of the heterostructure. This conforms with predictions from qualitative strain-energy considerations. The greatest density of misfit dislocations was present at the Si1-xGex interface between the bottom two layers of the heterostructure. Dislocations were observed to extend out of the interface and up into the heterolayer structure. The defects were found to interact with interfaces in the structure and finally cease extending upwards towards the surface of the wafer. In addition to graded heterolayers, SiGe heterolayers grown on SIMOX were also investigated. The structures consisted of epi-silicon grown on a Si/Si1-xGex superlattice which was in turn grown on a Si/SiO2 (SIMOX) structure. The behavior of defects in the layers was of interest. TEM characterization showed a large density of extended-defects present in the layers. Dislocations were observed to originate at the SIMOX oxide/Si interface, propagate up through the SiGe superlattice and into the epi-Si layer. Some dislocations were found to interact with the SiGe superlattice and cease propagating up towards the top of the wafer. SiGe superlattices with a higher concentration of Ge are more effective in reducing defect propagation towards the surface of the wafer.