As-synthesized single-walled carbon nanotubes (SWNTs) are mixtures of semiconducting and metallic species and separation of the two is of crucial importance for many applications. In this article, the methods employed for the enrichment of semiconducting and metallic SWNTs are presented, along with possible procedures to prepare either of the species selectively. Equally important are the methods for chirality selection. The discovery of metal-semiconductor transitions in SWNTs induced by interaction with electron donor and acceptor molecules is not only of academic interest, but may also find applications. Synthesis of Y-junction SWNTs with unique electronic properties at the junction is yet to be fully accomplished.