  Coulomb interaction effects have pronounced consequences in carbon nanotubes
due to their 1D nature. In particular, correlations imply the breakdown of
Fermi liquid theory and typically lead to Luttinger liquid behavior
characterized by pronounced power-law suppression of the transport current and
the density of states, and spin-charge separation. This paper provides a review
of the current understanding of non-Fermi liquid effects in metallic
single-wall nanotubes (SWNTs). We provide a self-contained theoretical
discussion of electron-electron interaction effects and show that the tunneling
density of states exhibits power-law behavior. The power-law exponent depends
on the interaction strength parameter $g$ and on the geometry of the setup.
  We then show that these features are observed experimentally by measuring the
tunneling conductance of SWNTs as a function of temperature and voltage. These
tunneling experiments are obtained by contacting metallic SWNTs to two
nanofabricated gold electrodes. Electrostatic force microscopy (EFM)
measurements show that the measured resistance is due to the contact resistance
from the transport barrier formed at the electrode/nanotube junction. These EFM
measurements show also the ballistic nature of transport in these SWNTs. While
charge transport can be nicely attributed to Luttinger liquid behavior,
spin-charge separation has not been observed so far. We briefly describe a
transport experiment that could provide direct evidence for spin-charge
separation.
