The microtubule associated protein, Tau, is implicated in a class of neurodegenerative diseases that are characterized by misregulation of axonal transport. While Tau is known to regulate microtubule dynamics, its exact role in both the normal and pathological state of the neuron is still unclear. Moreover, six isoforms of tau are expressed in adult human brain, but the isoform specificity of Tau's function is not completely understood either. To address these issues, we use a total internal reflection fluorescence (TIRF) microscopy based dynamic microtubule assay. We are currently working with 3RS- and 3RL- isoforms of Tau, which differ in the number of alternatively-expressed acidic inserts in the N-terminal region of the molecule. Consistent with previous studies, our results show that both of these isoforms reduce microtubule catastrophe frequencies. Interestingly, our results also show that only the 3RL-isoform increases microtubule rescue frequency. We also show that Tau reduces the rate at which microtubule tip structure evolves while it is growing, such that the microtubule tips become tapered at a slower rate in the presence of Tau compared to the no Tau control. This is a previously unknown mechanism by which Tau can alter the microtubule catastrophe frequency, and provides important new insights into the isoform-specific regulation of microtubule dynamics by Tau.