Arrays are used for many purposes, most prominently to measure level of gene expression for tens of thousands of genes simultaneously. Microarrays consist of probe DNA attached to the solid surface (chips), which is hybridized with fluorescent labeled cDNA samples. Microarrays can be classified into two categories. Oligonucleotide array uses 25 bp oligonucleotide probe DNA that is synthesized in situ on the solid surface photolithographically. Avidin-phycoerythrin is used to develop fluorescence. Compared to oligonucleotide array, cDNA microarray is more flexible to customize. The size of cDNA produced by RT-PCR, ranges from 250 to 750 bp. Spotter and twocolor scanner are needed for cDNA microarray. The ratio of Cy5/Cy3 is plotted and clustered for each gene to see the expression pattern. With the fast growing computer technologies and improved understanding for the genes, microarray is a promising tool to enhance our knowledge about genetically complicated diseases. We can choose the most effective drug to the specific pathogen for each person according to the expression pattern. Gene therapy would be applied prophylactically for the patients who have disease marker genes in the future. In spite of high price and technical subtleness of microarray, we had better to be accustomed to this system because we are living in the post-genomic era.