Gene therapy returns to the center stage of medicine to treat patients with diseases that are unable to be cured with the conventional therapeutic strategies. This development is due to various reasons, including vector development and significant achievement in next-generation sequencing. Among the various methodologies of gene therapy, nucleic acid-based therapy has been considered to be promising in various diseases. The development of delivery methods to target cells in vivo, however, remains critical. These include viral vector-based and nonviral vector-based gene delivery methods as well as physical approaches such as hydrodynamic gene delivery (HGD). HGD is a simple and effective in vivo gene transfer method for the functional analysis of therapeutic genes and regulatory elements in small animals. Moreover, this chapter outlines the principle of HGD, gene expression studies in rodents, and recent advances in clinical application of HGD and provides future perspectives in developing a safe and efficient method for nucleic acid-based therapy.