The intercostal muscles comprise many different groups of muscles that run between the ribs, and help form and move the chest wall.

The intercostal muscles are mainly involved in the mechanical aspect of breathing by helping expand and shrink the size of the chest cavity.

[1] There are three principal layers: Both the external and internal muscles are innervated by the intercostal nerves[3](the ventral rami of thoracic spinal nerves), are supplied by the intercostal arteries, and are drained by the intercostal veins.

Their fibers run in opposite directions.

The scalene muscles, which also move the chest wall and have a function in inhalation, are also intercostal muscles, just not one of the three principal layers.