The growth and properties of Si1-yCy. and Si1-x-yGexCy alloys pseudomorphically strained on Si(001) will be reviewed. Although the bulk solubility of carbon in silicon is small, epitaxial layers with more than 1% C can be fabricated by molecular beam epitaxy and various chemical vapor deposition techniques. One of the most crucial questions is the relation between substitutional and interstitial carbon incorporation, which has a large impact on the electrical and optical properties of these layers. The carbon substitutionality (fraction of substitutional incorporated carbon atoms) is strongly influenced by the growth conditions, such as growth temperature and Si growth rate. The mechanical and structural properties, and the influence of C atoms on band structure and charge carrier properties will be discussed. Further. we will show how lower carbon concentrations can influence dopant diffusion, without affecting strain and band alignment. We demonstrated that suppressed boron diffusion in carbon-rich epitaxial layers can be used to increase the performance of SiGe heterojunction bipolar transistors (HBTs). We demonstrate epitaxially grown SiGe:C HBTs with static and dynamic performance suitable for high frequency applications. Compared to SiGe technologies, the addition of carbon provides significantly greater flexibility in process design and offers wider latitude in process margins. The physical mechanism for suppressed boron diffusion in carbon-rich Si and SiGe (about 0.1% carbon) is an undersaturation of Si self-interstitials due to outdiffusion of carbon.