The vast amount of sequence information becoming available on genes from man and from other species calls for corresponding increases in the rate of collection for data of a more functional nature. Expression measurements often constitute a first step in this direction, and can be performed on a reasonably large scale using highly parallel hybridization methods. Large sets of targets (clones, inserts, oligonucleotides) are hybridized with labeled complex probes prepared from total cell or organ mRNA; under the proper conditions, signals measure the relative abundance of each sequence species, and can be acquired quantitatively. These techniques are presently available in three formats: high-density membranes to be hybridized with radioactive complex probes, microarrays of DNA spots (a miniaturized version of the former technique) using fluorescent complex probes, and oligonucleotide chips that, although developed originally for mutation detection, can be adapted to perform expression measurements. The miniaturized formats clearly represent the future, since they allow higher sensitivity, assay of large numbers of entities and hopefully provide the opportunity to use small amounts of starting material.