Dyneins are a family of cytoskeletal motor proteins (though they are actually protein complexes) that move along microtubules in cells. They convert the chemical energy stored in ATP to mechanical work. Dynein transports various cellular cargos, provides forces and displacements important in mitosis, and drives the beat of eukaryotic cilia and flagella. All of these functions rely on dynein's ability to move towards the minus-end of the microtubules, known as retrograde transport; thus, they are called "minus-end directed motors". In contrast, most kinesin motor proteins move toward the microtubules' plus-end, in what is called anterograde transport.

Dyneins can be divided into two groups: cytoplasmic dyneins and axonemal dyneins, which are also called ciliary or flagellar dyneins.

Axonemal dynein causes sliding of microtubules in the axonemes of cilia and flagella and is found only in cells that have those structures.

Cytoplasmic dynein, found in all animal cells and possibly plant cells as well, performs functions necessary for cell survival such as organelle transport and centrosome assembly. Cytoplasmic dynein moves processively along the microtubule; that is, one or the other of its stalks is always attached to the microtubule so that the dynein can "walk" a considerable distance along a microtubule without detaching.

Cytoplasmic dynein helps to position the Golgi complex and other organelles in the cell. It also helps transport cargo needed for cell function such as vesicles made by the endoplasmic reticulum, endosomes, and lysosomes (Karp, 2005). Dynein is involved in the movement of chromosomes and positioning the mitotic spindles for cell division. Dynein carries organelles, vesicles and possibly microtubule fragments along the axons of neurons toward the cell body in a process called retrograde axonal transport. Additionally, dynein motor is also responsible for the transport of degradative endosomes retrogradely in the dendrites.

Cytoplasmic dynein positions the spindle at the site of cytokinesis by anchoring to the cell cortex and pulling on astral microtubules emanating from centrosome. While a postdoctoral student at MIT, Tomomi Kiyomitsu discovered how dynein has a role as a motor protein in aligning the chromosomes in the middle of the cell during the metaphase of mitosis. Dynein pulls the microtubules and chromosomes to one end of the cell. When the end of the microtubules become close to the cell membrane, they release a chemical signal that punts the dynein to the other side of the cell. It does this repeatedly so the chromosomes end up in the center of the cell, which is necessary in mitosis. Budding yeast have been a powerful model organism to study this process and has shown that dynein is targeted to plus ends of astral microtubules and delivered to the cell cortex via an offloading mechanism.

Dynein and kinesin can both be exploited by viruses to mediate the viral replication process. Many viruses use the microtubule transport system to transport nucleic acid/protein cores to intracellular replication sites after invasion host the cell membrane. Not much is known about virus' motor-specific binding sites, but it is known that some viruses contain proline-rich sequences (that diverge between viruses) which, when removed, reduces dynactin binding, axon transport (in culture), and neuroinvasion in vivo. This suggests that proline-rich sequences may be a major binding site that co-opts dynein.

Each molecule of the dynein motor is a complex protein assembly composed of many smaller polypeptide subunits. Cytoplasmic and axonemal dynein contain some of the same components, but they also contain some unique subunits.