Microtubules are polymers of tubulin that form part of the cytoskeleton and provide structure and shape to eukaryotic cells. Microtubules can be as long as 50 micrometres, as wide as 23 to 27 nm and have an inner diameter between 11 and 15 nm. They are formed by the polymerization of a dimer of two globular proteins, alpha and beta tubulin into protofilaments that can then associate laterally to form a hollow tube, the microtubule. The most common form of a microtubule consists of 13 protofilaments in the tubular arrangement.

Microtubules play an important role in a number of cellular processes. They are involved in maintaining the structure of the cell and, together with microfilaments and intermediate filaments, they form the cytoskeleton. They also make up the internal structure of cilia and flagella. They provide platforms for intracellular transport and are involved in a variety of cellular processes, including the movement of secretory vesicles, organelles, and intracellular macromolecular assemblies. They are also involved in cell division (by mitosis and meiosis) and are the main constituents of mitotic spindles, which are used to pull eukaryotic chromosomes apart.

Microtubules are nucleated and organized by microtubule-organizing centres, such as the centrosome found in the center of many animal cells or the basal bodies of cilia and flagella, or the spindle pole bodies found in most fungi.

There are many proteins that bind to microtubules, including the motor proteins dynein and kinesin, microtubule-severing proteins like katanin, and other proteins important for regulating microtubule dynamics. Recently an actin-like protein has been found in the gram-positive bacterium Bacillus thuringiensis, which forms a microtubule-like structure called a nanotubule, involved in plasmid segregation. Other bacterial microtubules have a ring of five protofilaments.

Tubulin and microtubule-mediated processes, like cell locomotion, were seen by early microscopists, like Van Leeuwenhoek (1677). However, the fibrous nature of flagella and other structures were discovered two centuries later, with improved light microscopes, and confirmed in the 20th century with the electron microscope and biochemical studies.

In vitro assays for microtubule motor proteins such as dynein and kinesin are researched by fluorescently tagging a microtubule and fixing either the microtubule or motor proteins to a microscope slide, then visualizing the slide with video-enhanced microscopy to record the travel of the motor proteins. This allows the movement of the motor proteins along the microtubule or the microtubule moving across the motor proteins. Consequently, some microtubule processes can be determined by kymograph.

In eukaryotes, microtubules are long, hollow cylinders made up of polymerized α- and β-tubulin dimers. The inner space of the hollow microtubule cylinders is referred to as the lumen. The α and β-tubulin subunits are ~50% identical at the amino acid level, and both have a molecular weight of approximately 50 kDa.

These α/β-tubulin dimers polymerize end-to-end into linear protofilaments that associate laterally to form a single microtubule, which can then be extended by the addition of more α/β-tubulin dimers. Typically, microtubules are formed by the parallel association of thirteen protofilaments, although microtubules composed of fewer or more protofilaments have been observed in various species  as well as in vitro.