Gene therapy involves inserting normal genes into patients to compensate for mutated genes that cause disease. There are four main approaches: gene replacement, gene correction, gene regulation, or selective mutation reversal. The first gene therapy treated a girl with severe combined immunodeficiency by removing her white blood cells, inserting the missing gene, and returning the cells. While some early attempts showed promise, others led to patient deaths due to immune reactions. Current research focuses on developing safer viral and non-viral methods of gene delivery to address issues like short-term effects, immune responses, and control over insertion location. Recent successful human trials have treated immunodeficiency, muscular dystrophy, and Parkinson's disease.