The theoretical scientist attempting to model transport in modern nanoscale systems faces many challenges.

• The number of atoms requiring a microscopic treatment may vary from a few to several millions. The transport may be coherent, or dominated by interaction effects.
• No single formalism can capture all the different facets, and in this talk we attempt to give a birds-eye-view of the various theoretical techniques available in the literature.
• As the figure illustrates there are indeed many approaches available, with complementary strengths and weaknesses.
• Whenever possible, comparison with experiments will be made.