Tau modulates microtubule dynamics by increasing polymerization rate, decreasing the rate of transition into the shrinking phase (catastrophe), and inhibiting depolymerization. Tau strongly suppresses catastrophe rate, independent of its effect on elongation. Phosphorylation by MAP2 kinase reduces tau's affinity for microtubules, thereby decreasing its ability to modulate dynamics. Tau generates a partially stable but dynamic microtubule state, which is perturbed by phosphorylation. Tau's effects on microtubule assembly were studied using direct observation of microtubules, revealing that tau increases elongation rate, decreases catastrophe frequency, and reduces depolymerization rate. Phosphorylation by MAP2 kinase decreases tau's activity by lowering its affinity for microtubules, while CAM kinase phosphorylation has no effect. Tau's binding affinity for microtubules was measured, showing a higher dissociation constant for phosphorylated tau. Tau's effects on microtubule dynamics were further analyzed, showing that tau increases elongation rate, decreases catastrophe frequency, and reduces depolymerization rate. Phosphorylation by MAP2 kinase decreases tau's activity by lowering its affinity for microtubules, while CAM kinase phosphorylation has no effect. Tau's binding affinity for microtubules was measured, showing a higher dissociation constant for phosphorylated tau. Tau's effects on microtubule dynamics were further analyzed, showing that tau increases elongation rate, decreases catastrophe frequency, and reduces depolymerization rate. Phosphorylation by MAP2 kinase decreases tau's activity by lowering its affinity for microtubules, while CAM kinase phosphorylation has no effect.Tau modulates microtubule dynamics by increasing polymerization rate, decreasing the rate of transition into the shrinking phase (catastrophe), and inhibiting depolymerization. Tau strongly suppresses catastrophe rate, independent of its effect on elongation. Phosphorylation by MAP2 kinase reduces tau's affinity for microtubules, thereby decreasing its ability to modulate dynamics. Tau generates a partially stable but dynamic microtubule state, which is perturbed by phosphorylation. Tau's effects on microtubule assembly were studied using direct observation of microtubules, revealing that tau increases elongation rate, decreases catastrophe frequency, and reduces depolymerization rate. Phosphorylation by MAP2 kinase decreases tau's activity by lowering its affinity for microtubules, while CAM kinase phosphorylation has no effect. Tau's binding affinity for microtubules was measured, showing a higher dissociation constant for phosphorylated tau. Tau's effects on microtubule dynamics were further analyzed, showing that tau increases elongation rate, decreases catastrophe frequency, and reduces depolymerization rate. Phosphorylation by MAP2 kinase decreases tau's activity by lowering its affinity for microtubules, while CAM kinase phosphorylation has no effect. Tau's binding affinity for microtubules was measured, showing a higher dissociation constant for phosphorylated tau. Tau's effects on microtubule dynamics were further analyzed, showing that tau increases elongation rate, decreases catastrophe frequency, and reduces depolymerization rate. Phosphorylation by MAP2 kinase decreases tau's activity by lowering its affinity for microtubules, while CAM kinase phosphorylation has no effect.