High-resolution microscopic analysis has precisely revealed the control of microtubule dynamics by individual microtubule-associated proteins (MAPs) in vitro. Furthermore, transfection of MAP cDNA into fibroblasts and subsequent analysis using microinjection of caged fluorescein-labeled tubulin and photoactivation have enabled the function of MAPs in microtubule dynamics to be studied in detail in vivo. Systematic, quantitative studies using transfection of various kinds of MAP cDNA deletion mutants have demonstrated the complex mechanism for microtubule bundling in vivo, and have shown the involvement in microtubule bundling of both microtubule binding and projection regions of MAPs. A similar approach, combined with detailed structural analysis, has indicated clearly that differences in the amino-terminal projection region of MAPs can determine differential organization of MT bundles, and thus influence the characteristic organization of microtubule domains in dendrites and axons.