The means by which proteins and RNAs are exchanged between cytoplasm and nucleus have interested cell biologists for many years, and the field has seen a number of exciting recent advances. Much has been learned about the intricate architecture of the nuclear pore-complex, the mechanisms by which transport substrates are sorted, and the supply of energy for exchange processes. This book attempts a general review of the growing body of knowledge. However, the authors challenge the presumptions implicit in some interpretations of the evidence, re-examining the concept of "transport" within cells, and suggesting that lessons learned from nucleocytoplasmic transport studies can elucidate wider aspects of cell biology.