The human neuromuscular diseases X-linked myotubular myopathy and Charcot-Marie-Tooth disease type 4B are caused by mutations in myotubularin family proteins. The myotubularins are a unique subfamily of protein tyrosine phosphatases that utilize inositol phospholipids, rather than phosphoproteins, as substrates. Recent structural studies, including the first crystal structure of a myotubularin family protein, have defined the structural features that are characteristic of the family and revealed the molecular basis of their unique substrate specificity. Interestingly, the myotubularin family contains a subgroup of proteins that are catalytically inactive. Recent biochemical studies have established that the inactive myotubularins function as adaptors for the active members and play an important regulatory role within the family.