Gene therapy is a technique that aims at the delivery of nucleic acids to cells, to obtain a therapeutic effect. In situ gene therapy consists of the administration of the gene product to a specific site. It possesses several advantages, such as the reduction in potential side effects, the need for a lower vector dose, and, as a consequence, reduced costs, compared to intravenous administration. Different vectors, administration routes and doses involving in situ gene transfer have been tested both in animal models and humans, with in situ gene therapy drugs already approved in the market. In this review, we present applications of in situ gene therapy for different diseases, ranging from monogenic to multifactorial diseases, focusing mainly on therapies designed for the intra-articular and intraocular compartments, as well as gene therapies for the central nervous system (CNS) and for tumors. Gene therapy finally seems to blossom as a viable therapeutic approach. The growth in the number of clinical protocols shown here is evident, and the positive outcomes observed in several clinical trials indicate that more products based on in situ gene therapy should reach the market in the next years.