Comparative genomic hybridization (CGH) is a powerful molecular cytogenetic approach for identifying chromosomal abnormalities. CGH allows researchers to scan whole genomes for changes in DNA copy numbers. Starting in 2004, the array CGH became an irreplaceable method for the detection of gene mutations in people with congenital and developmental abnormalities, such as intellectual disability, dysmorphic characteristics, developmental delay, or several congenital deformities without an obvious syndrome pattern. This review focuses on the evolution of array CGH technology and its use in molecular diagnostics and its advantages over older cytogenetic tools. This review further highlights special arrays developed in the past decade which detect small intragenic copy number changes as well as large DNA segments for the region of heterozygosity.