Metalloproteinases MMP-2 and MMP-9 (also called gelatinases) are involved in cell invasion and in embryonic development and organogenesis. A growing number of reports suggest that MMP-2 and MMP-9 play some role in renal development, renal tubule physiology and glomerular pathophysiology. This editorial will focus on recent controversial data, especially those obtained from studies on MMP-9-deficient mice, which shed new light on the functions of gelatinases in normal and diseased kidneys.