This book presents selected topics about the path-integral method in Quantum Mechanics and Optics. Starting from an introduction to the grounds of functional integration theory, the main arguments of quantum and statistical mechanics, where the path-integral method works, are exposed. In particular, the partition function, the concept of instanton, tunneling and dissipative phenomena are analysed. The final section is devoted to selected and solved problems. This volume will be useful to beginners as well as more advanced students and researchers in the field.