Genetic modification refers to the process of manipulating the characteristics and functions of the original genes of an organism. Genetic engineering techniques have been applied in numerous fields including research, biotechnology, and medicine. Medicines such as insulin and human growth hormone are now produced in bacteria, experimental mice such as the oncomouse and the knockout mouse are being used for research purposes and insect resistant crops have been commercialized. Genetic engineering is also used to create animal models of human diseases. Also genetically modified pigs have been breed with the aim of increasing the success of pig to human organ transplantation. Gene therapy is the genetic engineering of humans by replacing defective human genes with functional copies. If the gene is inserted into the germline tissue it can be passed down to that person's descendants. Gene therapy has been used to treat patients suffering from immune deficiencies and trials have been carried out on other genetic disorders. This book gives a fundamental idea about tools used in genetic engineering, recombinant protein production and the development of a transgenic organism.