Interior permanent magnet synchronous machines (IPMSMs) are commonly utilized in many applications where high-torque density and low torque ripple are required. Flux barriers inside the rotor have a great impact on the electromagnetic performance and thus considered as an effective design parameter. This paper provides a comprehensive review of the function and design methodologies of flux barriers in IPMSMs. Both symmetric and asymmetric flux barriers that improve the torque capability and decrease torque ripples are discussed. The paper also investigates different flux barrier designs to reduce the stator and rotor iron losses to enhance the motor efficiency. The optimization of the shape of the flux barriers to mitigate irreversible demagnetization of the rotor magnets is also presented. It is concluded that a good design of flux barriers can significantly improve the motor's electromagnetic performance while reducing the manufacturing costs.