Microarrays have played a pivotal role in the early stages of omics data generation and analysis. For over 15 years, microarray technology dominated the field of transcriptomics, providing high-throughput platforms to measure gene expression levels across thousands of genes simultaneously. Microarrays enabled a wide range of genomic studies, leading to a vast repository of data in public databases such as the Gene Expression Omnibus (GEO). However, with the advent of next-generation sequencing (NGS), the use of microarrays has steadily declined. Despite this, the legacy of microarrays persists, particularly through the wealth of data generated over the years, which continues to fuel genomic research today. This chapter reviews the origins of microarrays and their impact on omics research and discusses the basic principles behind the technology, how it has evolved over the years, and how it lay the groundwork for the field as we know it today.