Some aspects of microwave transistor behavior are emerging as significant factors in applications involving increasing signal bandwidths, increasing carrier frequencies, and modulation schemes requiring high linearity. The design of circuits for these applications cannot rely on narrow-band, low frequency, or small-signal assumptions. Instead, a full spectral view of the frequency-dependent high-order nonlinearity of transistors needs to be considered. A paramount performance issue is intermodulation, which is significantly affected by circuit and device properties at base-band frequencies. Any variation in device or circuit behavior that responds to low frequencies is also excited by comparable differences between the frequencies of signal components. An exploration of the dependence of nonlinearity on signal spectra demonstrates the need to consider transistor behavior over all frequencies. To do this for the design of circuits and selection of device technology, accurate models of heating and charge trapping are essential. These require distributed network or sub-first order filters to model them because they impact an extremely large range of frequencies, from dc to many Gigahertz. The behavior of transistors varies with frequency and operating condition. This presents a measurement and characterization issue, which pulse testing coupled with alternative interpretation of RF data can ameliorate. A large-signal model should include trapping and heating and descriptions of nonlinearity that are continuous and consistent with small-signal behavior. The determination of trapping behavior in a closed-form description and its complete characterization are still a challenge.