The future trends of compound semiconductor and silicon-based millimeter-wave monolithic microwave integrated circuits (MMICs) and millimeter-wave system on chip (SOC), are discussed. The use of millimeter-wave MMICs has been increased for commercial applications including communications and automotive radar. The system in package (SIP) technology involves optimal combinations of the components for the best performance in a system. The SIP also requires that the circuit of the system can be implemented in different technologies and integrated into a single package. SOC technology involves that all radio-frequency front-end, analog, mixed-signal, and digital circuits need to be integrated on a single chip. GaAs and InP MMICs can be used to achieve excellent noise or power performance in the transceiver. Researchers are making efforts to develop millimeter wave SOC with base-band circuitry including digital, analog, or mixed-mode circuits.