This article reviews the electronic and transport properties of carbon nanotubes, focusing primarily on theoretical aspects but also discussing experimental results where appropriate. The authors begin by explaining how the metallic or semiconducting character of nanotubes is inferred from their topological structure using simple band-folding arguments. More sophisticated tight-binding and ab initio treatments are then introduced to discuss more subtle physical effects, such as those induced by curvature, tube-tube interactions, or topological defects. The same approach is followed for transport properties, covering fundamental aspects of conduction regimes and transport length scales using simple models of disorder. The article also emphasizes recent developments in semiempirical or ab initio simulations aimed at exploring the effect of realistic static scatterers and inelastic electron-phonon interactions. Specific issues beyond the noninteracting electron model, including excitonic effects, the Coulomb-blockade regime, and the Luttinger liquid, charge density waves, and superconducting transitions, are addressed. The content is theoretical, with experimental results used primarily to validate the theory. The review covers the structure of carbon nanotubes, their electronic properties, and transport properties, providing a comprehensive overview of the field.This article reviews the electronic and transport properties of carbon nanotubes, focusing primarily on theoretical aspects but also discussing experimental results where appropriate. The authors begin by explaining how the metallic or semiconducting character of nanotubes is inferred from their topological structure using simple band-folding arguments. More sophisticated tight-binding and ab initio treatments are then introduced to discuss more subtle physical effects, such as those induced by curvature, tube-tube interactions, or topological defects. The same approach is followed for transport properties, covering fundamental aspects of conduction regimes and transport length scales using simple models of disorder. The article also emphasizes recent developments in semiempirical or ab initio simulations aimed at exploring the effect of realistic static scatterers and inelastic electron-phonon interactions. Specific issues beyond the noninteracting electron model, including excitonic effects, the Coulomb-blockade regime, and the Luttinger liquid, charge density waves, and superconducting transitions, are addressed. The content is theoretical, with experimental results used primarily to validate the theory. The review covers the structure of carbon nanotubes, their electronic properties, and transport properties, providing a comprehensive overview of the field.