The grid oscillator is a planar structure that can combine a very large number of millimeterwave solid-state active devices. A 100-MESFET hybrid grid model is demonstrated at 5 GHz. The transistors in the grid self-lock with an effective radiated power (ERP) of 24 Watts and a conversion efficiency of 20%. An equivalent embedding circuit for a device in an infinite grid is found using symmetry. This equivalent circuit predicts the frequency of oscillation. Quasioptical injection-locking of the grid oscillator with an external signal has been demonstrated and analyzed.