Carbon nanotubes possess not only outstanding mechanical, electrical and thermal properties but also unique nonlinear optical properties. In this Feature Article, we review the recent developments of the carbon nanotube and its composites for nonlinear optical applications. Two research hot spots are discussed: optical limiting for laser protection and the saturable absorber as a mode-locking component for ultra-short pulsed lasers. The physical mechanism, materials and devices are intensively introduced for each application. All of the investigations indicate that the carbon nanotube is one of the main competitive candidates for next generation of photonic components.