We have used ultrahigh vacuum scanning tunneling microscopy (UHV STM) to study single-walled carbon nanotubes (SWNTs) supported by a hydrogen-terminated Si(100)-2×1 surface. Two distinct methods were used in the deposition of SWNTs. The first is an ex situ solution-based scheme, while the second involves direct mechanical transfer in situ. The UHV-prepared Si(100)-2×1:H surface is highly robust to ambient exposure, resulting in minimal surface degradation associated with ex situ chemical processing. We have achieved simultaneous atomic resolution STM images of SWNTs and the nearby silicon substrate. Local electronic characterization was performed using scanning tunneling spectroscopy (STS). I-V spectra depict a shift in the Fermi energy of semiconducting SWNTs towards the valence band edge, consistent with charge transfer between the nanotube and the silicon substrate. A metallic SWNT shows an approximately linear I-V response at low bias, while the surrounding Si(100)-2×1:H substrate shows a ∼1.3 eV gap. An advanced understanding of the fundamental physical and electronic interactions between carbon nanotubes and silicon will further the development of hybrid nanoscale devices and circuits.