Since the first edition of this book dedicated to differential display (DD) technology was published in 1997, we have witnessed an explosive interest in studying differential gene expression. The gene-hunting euphoria was initially powered by the invention of DD, which was gradually overtaken by DNA microarray technology in recent years. Then why is there still the need for second edition of this DD book? First of all, DD still enjoys a substantial lead over DNA microarrays in the ISI citation data (see Table 1), despite the h- dreds of millions of dollars spent each year on arrays. This may come as a surprise to many, but to us it implies that many of the DNA microarray studies went unpublished owing to their unfulfilled promises (1). Second, unlike DNA microarrays, DD is an "open"-ended gene discovery method that does not depend on prior genome sequence information of the organism being studied. As such, DD is applicable to the study of all living organisms-from bacteria, fungi, insects, fish, plants, to mammals-even when their genomes are not sequenced. Second, DD is more accessible technically and financially to most cost-conscious "cottage-industry" academic laboratories. So clearly DD still has its unique place in the modern molecular biological toolbox for gene expression analysis.