In eukaryotic cells, phosphatidylinositol is subject to differential phosphorylation, resulting in the production of seven distinct phosphatidylinositol phosphates, often referred to as phosphoinositides (PIs). PIs have numerous distinct roles in cellular regulation and membrane trafficking. Recently, myotubularin family PI 3-phosphatases have emerged as key regulators of phosphatidylinositol 3-phosphate and phosphatidylinositol 3,5-bisphosphate, two PIs that regulate traffic within the endosomal–lysosomal pathway. Mutations in several myotubularin genes lead to myotubular myopathy and Charcot-Marie-Tooth peripheral neuropathy. Strikingly, nearly half of the members of the human myotubularin family appear to be catalytically inactive. Several inactive myotubularins have essential functions in mammals. Recent work in mammalian cells and model organisms is shedding light on the roles of myotubularins in membrane traffic.