Several tau posttranslational modifications have been implicated in neuronal degeneration. Among them, tau fragmentation has been identified not only in brain samples obtained from Alzheimer's disease (AD) and related disorder subjects but also in AD culture and animal models. Some of these tau fragments have not been extensively studied. In contrast, data obtained recently showed that tau fragmentation mediated by enhanced or abnormal calpain, caspase 2, caspase 3, and asparagine endopeptidase activity results in the formation of toxic fragments. These cleaved tau forms induce neuronal death, synapse loss, and/or behavioral deficits. Here, we described protease activity assays and methods to study the effects of tau fragments on neuronal viability.