The wall of a microtubule is built from αβ-tubulin dimers, and highly purified αβ-tubulin will assemble in vitro into microtubules. But assembly of microtubules in vitro can be greatly enhanced by the presence of stabilizing microtubule-associated proteins (MAPs). Stabilizing MAPs represent just one class of proteins that interacts with tubulin in microtubules; other classes of MAPs destabilize microtubules or modify their growth properties. We discuss the various classes of MAPs in this section. The regulation of microtubule structure and dynamics is critical for proper cell function. As we will see later, microtubules are the major organizers of organelles in animal cells, and their stability and dynamics are tailored for the specific function of the cell at any given time. For example, the dynamics of microtubules increase dramatically as a cell enters mitosis to allow the cell to build a new configuration of microtubules, the mitotic spindle.