We present several design techniques to achieve high efficiency and linear power amplifiers in the millimeterwave frequencies. We will first review the performance of power amplifiers in different semiconductor process technologies at millimeter-wave frequencies. We will discuss the design, implementation and performance of stacked-FET power amplifiers, Doherty power amplifiers and linearization techniques to achieve high efficiency and linearity in millimeter-wave frequencies. The presented power amplifiers have applications in the 5G wireless communications.