In April 2003, the 50th anniversary year of the discovery of the double-helical structure of DNA, a high-quality and comprehensive sequencing of the human genome was completely accomplished, and a revolution in molecular cell biology and molecular medicine had begun. Many complex questions arise (e.g. how genes contribute to normal human development, individual variability, and common diseases) and they need complex answers. It was necessary to develop techniques that permit complex answers. Microarray technology is a revolution technology that allows the simultaneous assessment of the transcription of tens of thousands of genes rapidly, as well as of their relative expression between normal and injured cells. There is widespread hope that microarrays will significantly impact on our ability to explore the genetic changes associated with etiology and development of many diseases, and to discover new biomarkers for disease diagnosis and prognosis prediction, and new therapeutic tools. The present article provides an overview of microarray technology with accents on its recent advantages and limitations. The first chapter is focused on the basic principles of microarrays, array platforms and fabrication, and advantages and restrictions of cDNA-based (spotted) and oligonucleotide-based arrays (GeneChips and Codelink). The second chapter describes the crucial points in microarray study design (e.g. choice of reference source, sample preparation and preservation, labeling, and hybridization). Finally, a brief description of microarray data normalization, mining, and validation is given at the end of the review, transferring to several basic web sites and software with detailed explanation of this most underappreciated challenge facing researchers working on microarray projects.