Restriction enzymes also known as restriction endonucleases (REases) are the basic tools of molecular biology. They are enzymes that cleave double-stranded deoxyribonucleic acids (DNAs) in a sequence-specific manner and are ubiquitously present among prokaryotic organisms. They form part of the restriction-modification systems, which usually consist of an endonuclease and a methyltransferase. Restriction endonucleases have been the workhorse of molecular biology for the past 30 years. They catalyze the breakage of phosphodiester bonds on DNA backbones at specific sites and, together with their companion methyltransferases, are part of bacterial defense systems against the invasion of bacteriophages. Their working principle as well as their applications since their discovery has greatly impacted and enhanced genetic engineering which this review article hopes to elaborate.