Due to the inherently fast transport associated with tunneling, resonant tunneling diodes have been proposed for use in extremely high frequency oscillators. Transit time effects in these oscillators should be considered and can be exploited to improve their rf performance, as in a quantum well injection transit time (QWITT) diode. Small and large signal analyses of these transit time effects are presented, and their implications on the optimum design and maximum possible oscillation frequency of quantum well oscillators is discussed.