A physical description is given of the wave‐particle resonance which can occur when particles undergoing a ∇ Bdrift are in phase with a wave propagating across the magnetic field (i.e., &ohgr; ∼ k⋅V∇ B, where V∇ Bis the ∇ Bdrift velocity). Specifically, the physics of the energy exchange mechanism is discussed and a general equation for the wave damping/growth rate is derived based upon physical arguments. The theory is applied to the lower‐hybrid‐drift instability and collisionless trapped electron instability.