Millimeter-wave applications allow for high-speed data transfer due to the wide bandwidths availablein comparison to the overcrowded spectrum at lower frequencies. Unfortunately, atmospheric attenuation also increases generally with frequency, demanding higher output powers and/or multi-element arrays to transmit signals over the air. New gallium nitride (GaN) semiconductor processes operate with high power densities on the order of several watts per millimeter, and cutoff frequencies well beyond 1 GHz. In this thesis, several GaN integrated circuits are designed at frequencies from Ka throughW-band (26.5–110 GHz). Both commercially available and developing GaN technologies are employed, with 150 nm, 120 nm, 90 nm, and 40 nm process nodes. The thesis begins with monolithic microwave integrated circuits (MMICs) at the component level, by exploring the optimal biasing networks on chip for various active circuits. Several Ka-band amplifiers are then designed, measured, and analyzed. Comparisons between nonlinear foundry and scalable physics-based HEMT models are performed. The next three chapters focus on various control circuits, i.e. phase shifters, switches, and isolators. These circuits are generally very wideband, covering all or most of V andW-bands (50–110 GHz) and demonstrate high linearity. The level of integration increases in Chapters 7 and 8 with integrated systems designed on a single MMIC, containing both amplifiers and control circuits. First, two active circulators are presented, operating over V and W-bands with measured isolation comparable to commercial products. Finally, two W-band integrated transceiver front ends are described. The first is the most wideband GaN front end published to date. The second is the only full-duplex GaN front end published to date. Every circuit presented is characterized in measurement and simulation, with small signal, large signal, and noise measurements describing their performance.