The growth and properties of Si_1-yC_y and Si_1-x-y Ge_xC_y alloys pseudomorphically strained on Si(001) will be reviewed. Although the bulk solubility of carbon in silicon is small, epitaxial layers which more than 1 at.% C can be fabricated by MBE and different CVD 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. Substitutionally incorporated C atoms allow strain manipulation, including the growth of an inversely strained Si_1-x-yGe_xC_y layer. Local ordering effects due to atomic size differences and the growth on reconstructed surfaces, the mechanical and structural properties, and the influence of C atoms on band structure and charge carrier properties will be discussed. Further we will, haw lower carbon concentrations can influence dopant diffusion, without affecting strain and band alignment. Finally, we will present possible applications of this new semiconducting material.