Approaches to define patterns of gene expression have applications in a wide range of biological systems. The completion of the human genome project establishes a unique opportunity to understand the molecular control of different biological events through functional analysis. However, the huge database that currently exists will demand the utilization of high-throughput techniques for the assessment of multiple DNA sequences in a rapid and efficient manner. By determining genes with differential expression, it may be possible to decipher the mechanisms that underlie the control of different physiological pathways, which, in turn, may define future strategies to manage them. Although there exist a vast number of tools to screen genes for differential expression, we briefly examine here only some of those with regard to their advantages and disadvantages.