The importin α/β heterodimer targets hundreds of proteins to the nuclear-pore complex (NPC) and facilitates their translocation across the nuclear envelope. Importin α binds to classical nuclear localization signal (cNLS)-containing proteins and links them to importin β, the karyopherin that ferries the ternary complex through the NPC. A second karyopherin, the exportin CAS, recycles importin α back to the cytoplasm. In this article, we discuss control mechanisms that importin α exerts over the assembly and disassembly of the ternary complex and we describe how new groups of importin α genes arose during the evolution of metazoan animals to function in development and differentiation. We also describe activities of importin α that seem to be distinct from its housekeeping functions in nuclear transport.