One of the dogmas of transcriptional regulation in higher eukaryotes suggests the existence of transcriptional domains with no promoter-enhancer interactions between them. Specific regulatory elements, known as insulators, block the interaction between an enhancer and a promoter. Insulators are believed to act as transcription domain boundaries. The review considers general properties of well-known insulators identified in Drosophila and vertebrates. The mechanism of action of insulators and their role in the regulation of gene expression are discussed.