Genes are the chemical blueprints that determine an organism's traits. Moving genes from one organism to another transfers those traits. Genetic engineering refers to a set of technologies that are being used to change the genetic makeup of cells and move genes across species boundaries to produce novel organisms. The techniques involve highly sophisticated manipulations of genetic material and other biologically important chemicals. Genetic engineering has been said to be no different than ancient breeding methods but this is untrue. For a start, breeding or crossbreeding, or in-breeding all work by using the same species. In contrast genetic engineering allows us to combine fish, mouse, human and insect genes in the same person or animal. Genetic engineering makes the whole digital revolution look nothing. Digital technology changes what we do. Genetic engineering has the power to change who we are. Genetic engineering is used to create animal models of human diseases. This book presents the principles of genetic engineering and its associated techniques in details. Besides useful as a textbook to undergraduate and postgraduate students, it is useful to many experienced researchers as an informative reference source.