Optical second harmonic generation (SHG) from Si surfaces has the advantage of surface sensitivity and can be applied in-situ during epitaxial growth process such as gas source molecular beam epitaxy (GSMBE) or chemical vapour depositions (CVD). This enables a number of processes occurring on the surface during epitaxial growth to be investigated in real time. Two processes are of particular interest in the epitxial growth of Si and SiGe structures: (1) oscillatory processes on the surface due to layer-by-layer growth for growth rate measurements and (2) surface segregation of Ge during formation of Si/SiGe heterojunctions. In this paper, we report on the oscillatory SHG response from the Si(0 0 1) surface during homo-epitaxy of Si and variations of SHG response across the SiGe/Si heterojunction during GSMBE growth, The oscillatory response is attributed to the domain coverage variation during layer-by-layer growth analogous to the origin of reflectance anisotropy oscillations. The changes in SHG during formation of SiGe/Si heterojunction is attributed to the gradual variation of surface Ge concentration due to segregation of Ge. The contribution from buried interfaces is shown to be negligible at the photon energy used.