The chapter introduces the concept of quantum phase transitions, which occur at zero temperature when a non-thermal parameter like pressure, chemical composition, or magnetic field is changed. These transitions are driven by quantum fluctuations, a consequence of Heisenberg's uncertainty principle. The lecture notes provide a pedagogical introduction to quantum phase transitions, covering basic facts about phase transitions and critical behavior, the importance of quantum mechanics, and the relationship between quantum and classical transitions. The Ising model in a transverse field is discussed as a primary example, and the connection between quantum phase transitions and non-Fermi liquid behavior in itinerant electron systems is briefly explored. The chapter also delves into the physics near a quantum critical point, including the scaling behavior and the different types of phase diagrams that can arise. The Ising model in a transverse field is used to illustrate these concepts, and the chapter concludes with a discussion of non-Fermi liquid behavior in heavy fermion systems, highlighting the role of quantum phase transitions in understanding complex materials.The chapter introduces the concept of quantum phase transitions, which occur at zero temperature when a non-thermal parameter like pressure, chemical composition, or magnetic field is changed. These transitions are driven by quantum fluctuations, a consequence of Heisenberg's uncertainty principle. The lecture notes provide a pedagogical introduction to quantum phase transitions, covering basic facts about phase transitions and critical behavior, the importance of quantum mechanics, and the relationship between quantum and classical transitions. The Ising model in a transverse field is discussed as a primary example, and the connection between quantum phase transitions and non-Fermi liquid behavior in itinerant electron systems is briefly explored. The chapter also delves into the physics near a quantum critical point, including the scaling behavior and the different types of phase diagrams that can arise. The Ising model in a transverse field is used to illustrate these concepts, and the chapter concludes with a discussion of non-Fermi liquid behavior in heavy fermion systems, highlighting the role of quantum phase transitions in understanding complex materials.