Gene expression is the conversion of DNA into messenger RNA (mRNA), which are translated into proteins that perform critical functions of the cells. Microarray is a high-throughput technique for studying global gene expression profiling.  Microarray mainly works based on the principle of hybridization. Two major types of microarray chips are cDNA arrays and Oligonucleotide arrays. The principle, methodology, types and applications of microarrays are discussed in this review.  Apart  from studying  gene expression, microarrays are also used in studying Single Nucleotide Polymorphisms (SNPs), DNA methylation, micro RNA, copy number variations, etc. in the genome. Though there is a recent boom and excitement in genomic analysis using Next Generation Sequencing (NGS) methods and studying the whole genome expression profiling using RNAseq, it is difficult to substitute the microarrays in terms of diverse applications and cost effectiveness of the latter.