Phased array element RF front ends typically use single pole double throw (SPDT) switches or circulators with high isolation to prevent leakage of transmit energy into the receiver circuits. However, as phased-array designs scale to the millimeter-wave range, with high degrees of integration, the physical size and performance degradations associated with switches and circulators can present challenges in meeting system performance and size/weight/power (SWAP) requirements. This work demonstrates a loss-aware methodology for analysis and design of switchless transmit/receive (T/R) circuits. The methodology provides design insights and a practical, generally applicable approach for solving the multi-variable optimization problem of switchless power amplifier/low-noise amplifier (PALNA) matching networks, which present optimal matching impedances to both the power amplifier (PA) and the low noise amplifier (LNA) while maximizing power transfer efficiency and minimizing dissipative losses in each (transmit or receive) mode of operation. Three PALNA example designs at W-band are presented in this dissertation, each following a distinct design methodology. The first example design in 32SOI CMOS leverages PA and LNA circuits that already include 50 Ω matching networks at both input and output. The second example design in 8XP SiGe develops the PA and LNA circuits and integrates the PA output and LNA input matching networks into the PALNA matching network that connects the PA and the LNA. The third design in 32SOI CMOS leverages the loss-aware PALNA design methodology to develop a PALNA that achieves simulated maximum power added efficiency of 18 % in transmit and noise figure of 7.5 dB in receive at 94 GHz, which is beyond the published state-of-art for T/R circuits. In addition, for comparison purposes, this dissertation also presents an efficient, switch-based T/R circuit design in 32SOI CMOS technology, which achieves a simulated maximum power added efficiency of 15 % in transmit and noise figure of 6.5 dB in receive at 94 GHz, which is also beyond the published state-of-art for T/R circuits. / Doctor of Philosophy / In military and commercial applications, phased arrays are devices primarily used to achieve focusing and steering of transmitted or received electromagnetic energy. Phased arrays consist of many elements, each with an ability to both transmit and receive radio frequency (RF) signals. Each element incorporates a power amplifier (PA) for transmit and a low noise amplifier (LNA) for receive, which are typically connected using a single pole double throw (SPDT) switch or a circulator with high isolation to prevent leakage of transmit energy into the receiver circuits. However, as phased arrays exploit the latest technological advances in circuit integration and their frequencies of operation increase, physical size and performance degradations associated with switches and circulators can present challenges in meeting system performance and size/weight/power (SWAP) requirements. This dissertation provides a loss-aware methodology for analysis and design of switchless transmit/receive (T/R) circuits where the switches and circulators are replaced by carefully designed power amplifier/low-noise amplifier (PALNA) impedance matching networks. In the switchless T/R circuits, the design goals of maximum power efficiency and minimum noise in transmit and receive, respectively, are achieved through impedance matching that is optimal and low-loss in both modes of operation simultaneously. Three distinct PALNA example designs at W-band are presented in this dissertation, each following a distinct design methodology. With each new design, lessons learned are leveraged and design methodologies are enhanced. The first example design leverages already available PA and LNA circuits and connects them using 50 Ω transmission lines whose lengths are designed to guarantee optimum impedance match in receive and transmit mode of operation. The second example design develops new PA and LNA circuits and connects them using 50 Ω transmission lines whose lengths are designed to simultaneously achieve optimum impedance matching for maximum power efficiency in transmit mode of operation and lowest noise in receive mode of operation. The third design leverages a loss-aware PALNA design methodology, a multi-variable optimization procedure, to develop a PALNA that achieves simulated maximum power added efficiency of 18 % in transmit and noise figure of 7.5 dB in receive at 94 GHz, which is beyond the published state-of-art for T/R circuits. In addition, for comparison purposes with the third PALNA design, this dissertation also presents an efficient, switch-based T/R circuit design, which achieves a simulated maximum power added efficiency of 15 % in transmit and noise figure of 6.5 dB in receive at 94 GHz, which is also beyond the published state-of-art for T/R circuits.