Microarray-based analysis of gene expression requires the assembly of a complex system of biological reagents, hardware, software and laboratory and analytic procedures. A variety of reported methodologies and commercial products offer an often-confusing plethora of choices to groups contemplating commencing large-scale analysis of gene expression. Regardless of the methodology or technology chosen, there are, in fact, central necessities for successful gene expression analysis. This talk will survey core requirements for producing microarrays, using them to determine comparative levels of gene expression between samples and carrying out comparisons of expression levels across multiple samples. Emphasis will be focused on comparison of quantitative values obtained from microarray data and reproducibility of microarray results.