Gene therapy is a therapeutic technique in which nucleic acid is delivered into the cells of a patient as a drug to treat disease. The fundamental strategy in gene therapy is the use of DNA which when administered, reaches the damaged cells, enters the cell, and disrupts or expresses a protein. Multiple delivery techniques can be used for DNA delivery. Recent advances in the understanding of nuclease function have enabled direct DNA editing using CRISPR and zinc finger nucleases. Genes are incorporated into chromosomes with the help of vectors, which then express the nucleases that knock out and replace the genes in the chromosome. The vectors used in gene therapy can be recombinant viruses or naked DNA or DNA complexes. There is thus a great potential in using gene therapy and gene editing in treating genetic and viral diseases as well as cancer. The ever growing need of advanced diagnostic and therapeutic technology is the reason that has fueled the research in the field of gene therapy in recent times. The various studies that are constantly contributing towards advancing therapeutic strategies and mechanisms, and evolution of gene therapy are examined in detail. This book aims to serve as a resource guide for students and experts alike and contribute to the growth of the discipline.