Microarrays are one of the latest breakthroughs in experimental molecular biology, which allow monitoring of gene expression for tens of thousands of genes in parallel and are already producing huge amounts of valuable data. Microarray RNA expression on a genome-wide range is now a proven technology, although the idea of analysis of expression many genes in one sample is not new. Since the development of clone printing technology and oligonucleotide synthesis allowed to produce high density microarray. In this publication we provide the information about the technology, available detection systems and data analysis software. Comprehensive review of current or fundamental papers using microarray technology application in immunology, rheumatoid arthritis, oncology, cystis fibrosis research, primary pulmonary hypertension, psychiatry, and allergic airways inflammation is also included.