Although hot-wire chemical vapor deposition (HWCVD) involves far fewer species than plasma enhanced CVD, 'some physics and chemistry' has been used in the title because less is known than unknown regarding the hot-wire deposition processes, even for pure silane. I will concentrate on intrinsic a-Si:H deposition from SiH4 or SiH4/H-2 vapors, since this is the simplest and most studied system, and it has considerable practical importance. I will start with the reactions on the hot wire, a metal filament, then discuss the radical-vapor reactions and finally, the surface reactions that produce the film and its properties. This is not a review, and I will not attempt to cover all work that has been done to elucidate these processes.