Whistler mode waves are known to play an important role in radiation belt dynamics. Global models of radiation belt particle lifetimes based on quasilinear diffusion are effective tools if the location and occurrence of waves and distribution of energetic particles are known. Unfortunately, many aspects of whistler mode wave propagation are still not settled and new observations often challenge past assumptions. We focus on the effect of field aligned cold plasma density structures on whistler mode propagation. Even though knowledge of the existence of such structures predates the space age, their prevalence and contribution to global dynamics is still the subject of debate. A review of past work and recent results is used to motivate potential investigation efforts.