In this article the presence of threading and misfit dislocations in strained modulation-doped Si/SiGe heterostructures and their effect on electron mobility are investigated. It is found that the low-temperature electron mobility is sensitive to threading dislocations when their density exceeds 3 x 10(8) cm(-2), and decreases by two orders of magnitude when the threading dislocation density is 1 x 10(11) cm(-2). The room-temperature mobility is reduced under the same conditions by 10% and 50%, respectively. Misfit segments in the graded Ge-content buffer limit the mobility when the Si channel is 0.4 mu m or less away from that buffer. Misfit dislocations at the Si-channel bottom interface very strongly scatter the electrons in the channel once a continuous network of misfit segments is created.