Elevated intraocular pressure (IOP) is a major risk factor for the development and progression of glaucoma. Previous prospective, randomized, long-term studies have demonstrated the strength of IOP reduction in slowing the progression of disease. It is well known that IOP is not a fixed value but fluctuates considerably over time. Although there have been some studies on IOP fluctuation and the progression of glaucoma, whether IOP fluctuation is an independent risk factor for glaucomatous damage and disease progression remains controversial. In this article, we reviewed the definition of IOP fluctuation, and both the evidence and the speculation for and against the effect of IOP fluctuation on glaucoma progression. Although conclusions seem to vary from study to study, we considered that different studies examined different groups of patients, at different stages of disease, and at different IOP levels. Our conclusion is that these apparently disparate results are not conflicting, but rather can be viewed as complementary. In clinical care, we recommend the consideration of IOP "modulation" rather than just IOP "reduction" when glaucoma patients are treated. Quality-based IOP control may be more effective than quantity-based IOP reduction to prevent or retard disease progression.