Microarray experiments are being performed more widely than ever before, but even seasoned investigators can experience technical problems with hybridizations. This chapter provides guidelines for recognizing, rectifying, and avoiding common trouble areas. Specifically, it addresses frequent complications related to artifacts of printing, RNA sample preparation and quality, fluorophore labeling, hybridization conditions, and posthybridization washes. Emphasis is placed on investigating problems though a combination of appropriate controls and image analysis, where diagnostic plots of data quality are used to illustrate characteristics of acceptable and unsatisfactory hybridizations. This chapter also discusses resources available to microarray users hoping to improve the sensitivity and specificity of their experiments.