Phased array antennas provide a significant improvement in revisit rate in comparison to mechanically rotated antennas. This is of particular importance for RADAR applications, and phased array antennas are typically utilized for high performance RADAR applications of current interest. Transmit/receive (T/R) modules play a significant role in determining the performance, reliability, and cost of a phased array antenna. T/R modules also encompass a diverse set of technologies including semiconductors, packaging, and automated assembly and also present significant technical challenges. This presentation will discuss performance, cost, and reliability challenges associated with T/R modules along with associated emerging technologies.