Gene therapy has fascinated clinicians, scientists, and patients since decades ago because of its potential to treat a disease at the genetic level. This can be achieved in many ways, including replacing a disease-causing gene with a healthy copy. Gene therapy must overcome complex tissue and cellular barriers to introduce genetic modifications into the nucleus of the target cells to drive a high level of gene expression. Moreover, the genetically corrected cells must be present in significantly large numbers in the body to reverse the diseased condition and yet able to escape immunological recognition and survive in the long term to sustain the benefit. This paper reviews the early work as well as recent developments in gene therapy, including improvements to viral vectors and novel treatment strategies, diseases targeted, approved gene therapies, and current challenges and prospects for future progress.