First described in 1978 by Michael Smith at the University of British Columbia in Vancouver, site-directed mutagenesis (SDM) has become an invaluable tool for molecular biologists. Originally named oligonucleotide-directed mutagenesis, the first experiments made use of short, 12-nucleotide strands of synthetic DNA containing a mismatch to an intrinsic reporter gene of the bacteriophage Φ X174. Using these oligonucleotides, together with DNA polymerase I from E. coli and T4 DNA ligase, Smith and his team demonstrated that it was possible to introduce a permanent mutation in the circular DNA of the phage, resulting in a phenotypic change. In this booklet, we bring together a collection of influential papers that all depended on SDM for their success. As one of the most used techniques in molecular biology, it plays an often unacknowledged–but essential–role in many research projects.