Microtubules are cytoskeletal structures in the cytoplasm of eukaryotic cells, and their highly dynamic properties are essential to perform a wide variety of vital functions in cells. Microtubule growth proceeds through the endwise addition of nucleotide-bound tubulin molecules. It has largely been assumed that only tubulin dimers can incorporate into microtubules, and that the chemical state of the nucleotide is crucial for the incorporation. Recent observations reveal that both tubulin dimers and oligomers can add to microtubule ends and that the chemical state of the nucleotide is not decisive for tubulin addition. Together with structural studies of tubulin, these results show tubulin assembly polymorphism, which could play a crucial role in microtubule-dependent cellular functions.